# 3. Language Reference¶

This reference seeks to describe every construct in the Futhark language. It is not presented in a tutorial fashion, but rather intended for quick lookup and documentation of subtleties. For this reason, it is not written in a bottom-up manner, and some concepts may be used before they are fully defined. It is a good idea to have a basic grasp of Futhark (or some other functional programming language) before reading this reference. An ambiguous grammar is given for the full language. The text describes how ambiguities are resolved in practice (for example by applying rules of operator precedence).

This reference describes only the language itself. Documentation for the built-in prelude is available elsewhere.

## 3.1. Identifiers and Keywords¶

id::=`letter`

(`letter`

| "_" | "'")* | "_"`id`

quals::= (`id`

".")+qualid::=`id`

|`quals`

`id`

binop::=`opstartchar`

`opchar`

*qualbinop::=`binop`

|`quals`

`binop`

| "`"`qualid`

"`"fieldid::=`decimal`

|`id`

opstartchar::= "+" | "-" | "*" | "/" | "%" | "=" | "!" | ">" | "<" | "|" | "&" | "^"opchar::=`opstartchar`

| "."constructor::= "#"`id`

Many things in Futhark are named. When we are defining something, we
give it an unqualified name (id). When referencing something inside
a module, we use a qualified name (qualid). The constructor names
of a sum type are identifiers prefixed with `#`

, with no space
afterwards. The fields of a record are named with fieldid. Note
that a fieldid can be a decimal number. Futhark has three distinct
name spaces: terms, module types, and types. Modules (including
parametric modules) and values both share the term namespace.

## 3.2. Primitive Types and Values¶

literal::=`intnumber`

|`floatnumber`

| "true" | "false"

Boolean literals are written `true`

and `false`

. The primitive
types in Futhark are the signed integer types `i8`

, `i16`

,
`i32`

, `i64`

, the unsigned integer types `u8`

, `u16`

, `u32`

,
`u64`

, the floating-point types `f32`

, `f64`

, as well as
`bool`

. An `f32`

is always a single-precision float and a `f64`

is a double-precision float.

int_type::= "i8" | "i16" | "i32" | "i64" | "u8" | "u16" | "u32" | "u64"float_type::= "f32" | "f64"

Numeric literals can be suffixed with their intended type. For
example `42i8`

is of type `i8`

, and `1337e2f64`

is of type
`f64`

. If no suffix is given, the type of the literal will be
inferred based on its use. If the use is not constrained, integral
literals will be assigned type `i32`

, and decimal literals type
`f64`

. Hexadecimal literals are supported by prefixing with `0x`

,
and binary literals by prefixing with `0b`

.

Floats can also be written in hexadecimal format such as `0x1.fp3`

,
instead of the usual decimal notation. Here, `0x1.f`

evaluates to
`1 15/16`

and the `p3`

multiplies it by `2^3 = 8`

.

intnumber::= (`decimal`

|`hexadecimal`

|`binary`

) [`int_type`

]decimal::=`decdigit`

(`decdigit`

|"_")*hexadecimal::= 0 ("x" | "X")`hexdigit`

(`hexdigit`

|"_")*binary::= 0 ("b" | "B")`bindigit`

(`bindigit`

| "_")*

floatnumber::= (`pointfloat`

|`exponentfloat`

|`hexadecimalfloat`

) [`float_type`

]pointfloat::= [`intpart`

]`fraction`

exponentfloat::= (`intpart`

|`pointfloat`

)`exponent`

hexadecimalfloat::= 0 ("x" | "X")`hexintpart`

`hexfraction`

("p"|"P") ["+" | "-"]`decdigit`

+intpart::=`decdigit`

(`decdigit`

|"_")*fraction::= "."`decdigit`

(`decdigit`

|"_")*hexintpart::=`hexdigit`

(`hexdigit`

| "_")*hexfraction::= "."`hexdigit`

(`hexdigit`

|"_")*exponent::= ("e" | "E") ["+" | "-"]`decdigit`

+

decdigit::= "0"..."9"hexdigit::=`decdigit`

| "a"..."f" | "A"..."F"bindigit::= "0" | "1"

### 3.2.1. Compound Types and Values¶

type::=`qualid`

|`array_type`

|`tuple_type`

|`record_type`

|`sum_type`

|`function_type`

|`type_application`

Compound types can be constructed based on the primitive types. The Futhark type system is entirely structural, and type abbreviations are merely shorthands. The only exception is abstract types whose definition has been hidden via the module system (see Module System).

tuple_type::= "(" ")" | "("`type`

(","`type`

)+ ")"

A tuple value or type is written as a sequence of comma-separated
values or types enclosed in parentheses. For example, `(0, 1)`

is a
tuple value of type `(i32,i32)`

. The elements of a tuple need not
have the same type – the value `(false, 1, 2.0)`

is of type
`(bool, i32, f64)`

. A tuple element can also be another tuple, as
in `((1,2),(3,4))`

, which is of type `((i32,i32),(i32,i32))`

. A
tuple cannot have just one element, but empty tuples are permitted,
although they are not very useful. Empty tuples are written `()`

and are of type `()`

.

array_type::= "[" [`dim`

] "]"`type`

dim::=`qualid`

|`decimal`

An array value is written as a sequence of zero or more
comma-separated values enclosed in square brackets: `[1,2,3]`

. An
array type is written as `[d]t`

, where `t`

is the element type of
the array, and `d`

is an integer or variable indicating the size.
We can often elide `d`

and write just `[]`

(an *anonymous size*),
in which case the size will be inferred. As an example, an array of
three integers could be written as `[1,2,3]`

, and has type
`[3]i32`

. An empty array is written as `[]`

, and its type is
inferred from its use. When writing Futhark values for such uses as
`futhark test`

(but not when writing programs), empty arrays are
written `empty([0]t)`

for an empty array of type `[0]t`

. When
using `empty`

, all dimensions must be given a size, and at least one
must be zero, e.g. `empty([2][0]i32)`

.

Multi-dimensional arrays are supported in Futhark, but they must be
*regular*, meaning that all inner arrays must have the same shape.
For example, `[[1,2], [3,4], [5,6]]`

is a valid array of type
`[3][2]i32`

, but `[[1,2], [3,4,5], [6,7]]`

is not, because there
we cannot come up with integers `m`

and `n`

such that
`[m][n]i32`

describes the array. The restriction to regular arrays
is rooted in low-level concerns about efficient compilation. However,
we can understand it in language terms by the inability to write a
type with consistent dimension sizes for an irregular array value. In
a Futhark program, all array values, including intermediate (unnamed)
arrays, must be typeable.

sum_type::=`constructor`

`type`

* ("|"`constructor`

`type`

*)*

Sum types are anonymous in Futhark, and are written as the
constructors separated by vertical bars. Each constructor consists of
a `#`

-prefixed *name*, followed by zero or more types, called its
*payload*. **Note:** The current implementation of sum types is
fairly inefficient, in that all possible constructors of a sum-typed
value will be resident in memory. Avoid using sum types where
multiple constructors have large payloads.

record_type::= "{" "}" | "{"`fieldid`

":"`type`

(","`fieldid`

":"`type`

)* "}"

Records are mappings from field names to values, with the field names known statically. A tuple behaves in all respects like a record with numeric field names starting from zero, and vice versa. It is an error for a record type to name the same field twice.

type_application::=`type`

`type_arg`

| "*"`type`

type_arg::= "[" [`dim`

] "]" |`type`

A parametric type abbreviation can be applied by juxtaposing its name and its arguments. The application must provide as many arguments as the type abbreviation has parameters - partial application is presently not allowed. See Type Abbreviations for further details.

function_type::=`param_type`

"->"`type`

param_type::=`type`

| "("`id`

":"`type`

")"

Functions are classified via function types, but they are not fully first class. See Higher-order functions for the details.

stringlit::= '"'`stringchar`

'"'stringchar::= <any source character except "\" or newline or quotes>

String literals are supported, but only as syntactic sugar for UTF-8
encoded arrays of `u8`

values. There is no character type in
Futhark.

## 3.3. Declarations¶

A Futhark file or module consists of a sequence of declarations. Each declaration is processed in order, and a declaration can only refer to names bound by preceding declarations.

dec::=`fun_bind`

|`val_bind`

|`type_bind`

|`mod_bind`

|`mod_type_bind`

| "open"`mod_exp`

| "import"`stringlit`

| "local"`dec`

| "#[" attr "]" dec

The `open`

declaration brings names defined in another module into
scope (see also Module System). For the meaning of `import`

, see
Referring to Other Files. If a declaration is prefixed with
`local`

, whatever names it defines will *not* be visible outside the
current module. In particular `local open`

is used to bring names
from another module into scope, without making those names available
to users of the module being defined. In most cases, using module
type ascription is a better idea.

### 3.3.1. Declaring Functions and Values¶

fun_bind::= ("let" | "entry") (`id`

| "("`binop`

")")`type_param`

*`pat`

+ [":"`type`

] "="`exp`

| ("let" | "entry")`pat`

`binop`

`pat`

[":"`type`

] "="`exp`

val_bind::= "let"`id`

[":"`type`

] "="`exp`

Functions and values must be defined before they are used. A function declaration must specify the name, parameters, and body of the function:

```
let name params...: rettype = body
```

Hindley-Milner-style type inference is supported. A parameter may be
given a type with the notation `(name: type)`

. Functions may not be
recursive. You may put *size annotations* in the return type and
parameter types; see Size Types. A function can be *polymorphic*
by using type parameters, in the same way as for Type
Abbreviations:

```
let reverse [n] 't (xs: [n]t): [n]t = xs[::-1]
```

Type parameters for a function do not need to cover the types of all parameters. The type checker will add more if necessary. For example, the following is well typed:

```
let pair 'a (x: a) y = (x, y)
```

A new type variable will be invented for the parameter `y`

.

Shape and type parameters are not passed explicitly when calling
function, but are automatically derived. If an array value *v* is
passed for a type parameter *t*, all other arguments passed of type
*t* must have the same shape as *v*. For example, consider the following
definition:

```
let pair 't (x: t) (y: t) = (x, y)
```

The application `pair [1] [2,3]`

will fail at run-time.

To simplify the handling of in-place updates (see In-place Updates), the value returned by a function may not alias any global variables.

### 3.3.2. User-Defined Operators¶

Infix operators are defined much like functions:

```
let (p1: t1) op (p2: t2): rt = ...
```

For example:

```
let (a:i32,b:i32) +^ (c:i32,d:i32) = (a+c, b+d)
```

We can also define operators by enclosing the operator name in parentheses and suffixing the parameters, as an ordinary function:

```
let (+^) (a:i32,b:i32) (c:i32,d:i32) = (a+c, b+d)
```

This is necessary when defining a polymorphic operator.

A valid operator name is a non-empty sequence of characters chosen
from the string `"+-*/%=!><&^"`

. The fixity of an operator is
determined by its first characters, which must correspond to a
built-in operator. Thus, `+^`

binds like `+`

, whilst `*^`

binds
like `*`

. The longest such prefix is used to determine fixity, so
`>>=`

binds like `>>`

, not like `>`

.

It is not permitted to define operators with the names `&&`

or
`||`

(although these as prefixes are accepted). This is because a
user-defined version of these operators would not be short-circuiting.
User-defined operators behave exactly like ordinary functions, except
for being infix.

A built-in operator can be shadowed (i.e. a new `+`

can be defined).
This will result in the built-in polymorphic operator becoming
inaccessible, except through the `intrinsics`

module.

An infix operator can also be defined with prefix notation, like an ordinary function, by enclosing it in parentheses:

```
let (+) (x: i32) (y: i32) = x - y
```

This is necessary when defining operators that take type or shape parameters.

### 3.3.3. Entry Points¶

Apart from declaring a function with the keyword `let`

, it can also
be declared with `entry`

. When the Futhark program is compiled any
top-level function declared with `entry`

will be exposed as an entry
point. If the Futhark program has been compiled as a library, these
are the functions that will be exposed. If compiled as an executable,
you can use the `--entry-point`

command line option of the generated
executable to select the entry point you wish to run.

Any top-level function named `main`

will always be considered an
entry point, whether it is declared with `entry`

or not.

The name of an entry point must not contain an apostrophe (`'`

),
even though that is normally permitted in Futhark identifiers.

### 3.3.4. Value Declarations¶

A named value/constant can be declared as follows:

```
let name: type = definition
```

The definition can be an arbitrary expression, including function calls and other values, although they must be in scope before the value is defined. A constant value may not have a unique type (see In-place updates). If the return type contains any anonymous sizes (see Size types), new existential sizes will be constructed for them.

### 3.3.5. Type Abbreviations¶

type_bind::= "type" ["^" | "~"]`id`

`type_param`

* "="`type`

type_param::= "["`id`

"]" | "'"`id`

| "'~"`id`

| "'^"`id`

Type abbreviations function as shorthands for the purpose of
documentation or brevity. After a type binding `type t1 = t2`

, the
name `t1`

can be used as a shorthand for the type `t2`

. Type
abbreviations do not create distinct types: the types `t1`

and
`t2`

are entirely interchangeable.

If the right-hand side of a type contains anonymous sizes, it must be
declared “size-lifted” with `type~`

. If it (potentially) contains a
function, it must be declared “fully lifted” with `type^`

. A lifted
type can also contain anonymous sizes. Lifted types cannot be put in
arrays. Fully lifted types cannot be returned from conditional or
loop expressions.

A type abbreviation can have zero or more parameters. A type
parameter enclosed with square brackets is a *size parameter*, and
can be used in the definition as an array dimension size, or as a
dimension argument to other type abbreviations. When passing an
argument for a shape parameter, it must be enclosed in square
brackets. Example:

```
type two_intvecs [n] = ([n]i32, [n]i32)
let x: two_intvecs [2] = (iota 2, replicate 2 0)
```

Size parameters work much like shape declarations for arrays. Like shape declarations, they can be elided via square brackets containing nothing. All size parameters must be used in the definition of the type abbreviation.

A type parameter prefixed with a single quote is a *type parameter*.
It is in scope as a type in the definition of the type abbreviation.
Whenever the type abbreviation is used in a type expression, a type
argument must be passed for the parameter. Type arguments need not be
prefixed with single quotes:

```
type two_vecs [n] 't = ([n]t, [n]t)
type two_intvecs [n] = two_vecs [n] i32
let x: two_vecs [2] i32 = (iota 2, replicate 2 0)
```

A *size-lifted type parameter* is prefixed with `'~`

, and a *fully
lifted type parameter* with `'^`

. These have the same rules and
restrictions as lifted type abbreviations.

## 3.4. Expressions¶

Expressions are the basic construct of any Futhark program. An
expression has a statically determined *type*, and produces a *value*
at runtime. Futhark is an eager/strict language (“call by value”).

The basic elements of expressions are called *atoms*, for example
literals and variables, but also more complicated forms.

atom::=`literal`

|`qualid`

("."`fieldid`

)* |`stringlit`

| "(" ")" | "("`exp`

")" ("."`fieldid`

)* | "("`exp`

(","`exp`

)* ")" | "{" "}" | "{" field (","`field`

)* "}" |`qualid`

"["`index`

(","`index`

)* "]" | "("`exp`

")" "["`index`

(","`index`

)* "]" |`quals`

"." "("`exp`

")" | "["`exp`

(","`exp`

)* "]" | "["`exp`

[".."`exp`

] "..."`exp`

"]" | "("`qualbinop`

")" | "("`exp`

`qualbinop`

")" | "("`qualbinop`

`exp`

")" | "(" ( "."`field`

)+ ")" | "(" "." "["`index`

(","`index`

)* "]" ")"exp::=`atom`

|`exp`

`qualbinop`

`exp`

|`exp`

`exp`

|`constructor`

`exp`

* |`exp`

":"`type`

|`exp`

":>"`type`

|`exp`

[ ".."`exp`

] "..."`exp`

|`exp`

[ ".."`exp`

] "..<"`exp`

|`exp`

[ ".."`exp`

] "..>"`exp`

| "if"`exp`

"then"`exp`

"else"`exp`

| "let"`pat`

"="`exp`

"in"`exp`

| "let"`id`

"["`index`

(","`index`

)* "]" "="`exp`

"in"`exp`

| "let"`id`

`type_param`

*`pat`

+ [":"`type`

] "="`exp`

"in"`exp`

| "(" "\"`pat`

+ [":"`type`

] "->"`exp`

")" | "loop"`pat`

[("="`exp`

)]`loopform`

"do"`exp`

| "#[" attr "]"`exp`

| "unsafe"`exp`

| "assert"`atom`

`atom`

|`exp`

"with" "["`index`

(","`index`

)* "]" "="`exp`

|`exp`

"with"`fieldid`

("."`fieldid`

)* "="`exp`

| "match"`exp`

("case"`pat`

"->"`exp`

)+field::=`fieldid`

"="`exp`

|`id`

pat::=`id`

|`literal`

| "_" | "(" ")" | "("`pat`

")" | "("`pat`

(","`pat`

)+ ")" | "{" "}" | "{"`fieldid`

["="`pat`

] (","`fieldid`

["="`pat`

])* "}" |`constructor`

`pat`

* |`pat`

":"`type`

loopform::= "for"`id`

"<"`exp`

| "for"`pat`

"in"`exp`

| "while"`exp`

index::=`exp`

[":" [`exp`

]] [":" [`exp`

]] | [`exp`

] ":"`exp`

[":" [`exp`

]] | [`exp`

] [":"`exp`

] ":" [`exp`

]

Some of the built-in expression forms have parallel semantics, but it is not guaranteed that the the parallel constructs in Futhark are evaluated in parallel, especially if they are nested in complicated ways. Their purpose is to give the compiler as much freedom and information is possible, in order to enable it to maximise the efficiency of the generated code.

### 3.4.1. Resolving Ambiguities¶

The above grammar contains some ambiguities, which in the concrete implementation is resolved via a combination of lexer and grammar transformations. For ease of understanding, they are presented here in natural text.

An expression

`x.y`

may either be a reference to the name`y`

in the module`x`

, or the field`y`

in the record`x`

. Modules and values occupy the same name space, so this is disambiguated by the type of`x`

.A type ascription (

`exp : type`

) cannot appear as an array index, as it conflicts with the syntax for slicing.In

`f [x]`

, there is am ambiguity between indexing the array`f`

at position`x`

, or calling the function`f`

with the singleton array`x`

. We resolve this the following way:If there is a space between

`f`

and the opening bracket, it is treated as a function application.Otherwise, it is an array index operation.

An expression

`(-x)`

is parsed as the variable`x`

negated and enclosed in parentheses, rather than an operator section partially applying the infix operator`-`

.The following table describes the precedence and associativity of infix operators. All operators in the same row have the same precedence. The rows are listed in increasing order of precedence. Note that not all operators listed here are used in expressions; nevertheless, they are still used for resolving ambiguities.

**Associativity****Operators**left

`,`

left

`:`

,`:>`

left

`||`

left

`&&`

left

`<=`

`>=`

`>`

`<`

`==`

`!=`

left

`&`

`^`

`|`

left

`<<`

`>>`

left

`+`

`-`

left

`*`

`/`

`%`

`//`

`%%`

left

`|>`

right

`<|`

right

`->`

left

juxtaposition

### 3.4.2. Patterns¶

We say that a pattern is *irrefutable* if it can never fail to match a
value of the appropriate type. Concretely, this means that it does
not require any specific sum type constructor (unless the type in
question has only a single constructor), or any specific numeric or
boolean literal. Patterns used in function parameters and `let`

bindings must be irrefutable. Patterns used in `case`

need not be
irrefutable.

A pattern `_`

matches any value. A pattern consisting of a literal
value (e.g. a numeric constant) matches exactly that value.

### 3.4.3. Semantics of Simple Expressions¶

#### 3.4.3.1. literal¶

Evaluates to itself.

#### 3.4.3.2. qualid¶

A variable name; evaluates to its value in the current environment.

#### 3.4.3.3. stringlit¶

Evaluates to an array of type `[]u8`

that contains the characters
encoded as UTF-8.

#### 3.4.3.4. `()`

¶

Evaluates to an empty tuple.

#### 3.4.3.5. `( e )`

¶

Evaluates to the result of `e`

.

#### 3.4.3.6. `(e1, e2, ..., eN)`

¶

Evaluates to a tuple containing `N`

values. Equivalent to the
record literal `{0=e1, 1=e2, ..., N-1=eN}`

.

#### 3.4.3.7. `{f1, f2, ..., fN}`

¶

A record expression consists of a comma-separated sequence of *field
expressions*. Each field expression defines the value of a field in
the record. A field expression can take one of two forms:

`f = e`

: defines a field with the name`f`

and the value resulting from evaluating`e`

.

`f`

: defines a field with the name`f`

and the value of the variable`f`

in scope.

Each field may only be defined once.

#### 3.4.3.8. `a[i]`

¶

Return the element at the given position in the array. The index may be a comma-separated list of indexes instead of just a single index. If the number of indices given is less than the rank of the array, an array is returned. The index may be of any unsigned integer type.

The array `a`

must be a variable name or a parenthesised expression.
Furthermore, there *may not* be a space between `a`

and the opening
bracket. This disambiguates the array indexing `a[i]`

, from ```
a
[i]
```

, which is a function call with a literal array.

#### 3.4.3.9. `a[i:j:s]`

¶

Return a slice of the array `a`

from index `i`

to `j`

, the
former inclusive and the latter exclusive, taking every `s`

-th
element. The `s`

parameter may not be zero. If `s`

is negative,
it means to start at `i`

and descend by steps of size `s`

to `j`

(not inclusive). Slicing can be done only with expressions of type
`i64`

.

It is generally a bad idea for `s`

to be non-constant.
Slicing of multiple dimensions can be done by separating with commas,
and may be intermixed freely with indexing.

If `s`

is elided it defaults to `1`

. If `i`

or `j`

is elided, their
value depends on the sign of `s`

. If `s`

is positive, `i`

become `0`

and `j`

become the length of the array. If `s`

is negative, `i`

becomes
the length of the array minus one, and `j`

becomes minus one. This means that
`a[::-1]`

is the reverse of the array `a`

.

In the general case, the size of the array produced by a slice is unknown (see Size types). In a few cases, the size is known statically:

`a[0:n]`

has size`n`

`a[:n]`

has size`n`

`a[0:n:1]`

has size`n`

`a[:n:1]`

has size`n`

This holds only if `n`

is a variable or constant.

#### 3.4.3.10. `[x, y, z]`

¶

Create an array containing the indicated elements. Each element must have the same type and shape.

#### 3.4.3.11. `x..y...z`

¶

Construct a signed integer array whose first element is `x`

and
which proceeds with a stride of `y-x`

until reaching `z`

(inclusive). The `..y`

part can be elided in which case a stride of
1 is used. A run-time error occurs if `z`

is less than `x`

or
`y`

, or if `x`

and `y`

are the same value.

In the general case, the size of the array produced by a range is unknown (see Size types). In a few cases, the size is known statically:

`1..2...n`

has size`n`

This holds only if `n`

is a variable or constant.

#### 3.4.3.12. `x..y..<z`

¶

Construct a signed integer array whose first elements is `x`

, and
which proceeds upwards with a stride of `y-x`

until reaching `z`

(exclusive). The `..y`

part can be elided in which case a stride of
1 is used. A run-time error occurs if `z`

is less than `x`

or
`y`

, or if `x`

and `y`

are the same value.

`0..1..<n`

has size`n`

`0..<n`

has size`n`

This holds only if `n`

is a variable or constant.

#### 3.4.3.13. `x..y..>z`

¶

Construct a signed integer array whose first elements is `x`

, and
which proceeds downwards with a stride of `y-x`

until reaching `z`

(exclusive). The `..y`

part can be elided in which case a stride of
-1 is used. A run-time error occurs if `z`

is greater than `x`

or
`y`

, or if `x`

and `y`

are the same value.

#### 3.4.3.14. `e.f`

¶

Access field `f`

of the expression `e`

, which must be a record or
tuple.

#### 3.4.3.15. `m.(e)`

¶

Evaluate the expression `e`

with the module `m`

locally opened, as
if by `open`

. This can make some expressions easier to read and
write, without polluting the global scope with a declaration-level
`open`

.

#### 3.4.3.16. `x`

*binop* `y`

¶

Apply an operator to `x`

and `y`

. Operators are functions like
any other, and can be user-defined. Futhark pre-defines certain
“magical” *overloaded* operators that work on many different types.
Overloaded functions cannot be defined by the user. Both operands
must have the same type. The predefined operators and their semantics
are:

`**`

Power operator, defined for all numeric types.

`//`

,`%%`

Division and remainder on integers, with rounding towards zero.

`*`

,`/`

,`%`

,`+`

,`-`

The usual arithmetic operators, defined for all numeric types. Note that

`/`

and`%`

rounds towards negative infinity when used on integers - this is different from in C.

`^`

,`&`

,`|`

,`>>`

,`<<`

Bitwise operators, respectively bitwise xor, and, or, arithmetic shift right and left, and logical shift right.

Shifting is undefined if the right operand is negative, or greater than or equal to the length in bits of the left operand.Note that, unlike in C, bitwise operators have

higherpriority than arithmetic operators. This means that`x & y == z`

is understood as`(x & y) == z`

, rather than`x & (y == z)`

as it would in C. Note that the latter is a type error in Futhark anyhow.

`==`

,`!=`

Compare any two values of builtin or compound type for equality.

`<`

,`<=`

.`>`

,`>=`

Company any two values of numeric type for equality.

#### 3.4.3.17. `x && y`

¶

Short-circuiting logical conjunction; both operands must be of type
`bool`

.

#### 3.4.3.18. `x || y`

¶

Short-circuiting logical disjunction; both operands must be of type
`bool`

.

#### 3.4.3.19. `f x`

¶

Apply the function `f`

to the argument `x`

.

#### 3.4.3.20. `#c x y z`

¶

Apply the sum type constructor `#x`

to the payload `x`

, `y`

, and
`z`

. A constructor application is always assumed to be saturated,
i.e. its entire payload provided. This means that constructors may
not be partially applied.

#### 3.4.3.21. `e : t`

¶

Annotate that `e`

is expected to be of type `t`

, failing with a
type error if it is not. If `t`

is an array with shape
declarations, the correctness of the shape declarations is checked at
run-time.

Due to ambiguities, this syntactic form cannot appear as an array
index expression unless it is first enclosed in parentheses. However,
as an array index must always be of type `i64`

, there is never a
reason to put an explicit type ascription there.

#### 3.4.3.22. `e :> t`

¶

Coerce the size of `e`

to `t`

. The type of `t`

must match the
type of `e`

, except that the sizes may be statically different. At
run-time, it will be verified that the sizes are the same.

#### 3.4.3.23. `! x`

¶

Logical negation if `x`

is of type `bool`

. Bitwise negation if
`x`

is of integral type.

#### 3.4.3.24. `- x`

¶

Numerical negation of `x`

, which must be of numeric type.

#### 3.4.3.25. `#[attr] e`

¶

Apply the given attribute to the expression. Attributes are an ad-hoc and optional mechanism for providing extra information, directives, or hints to the compiler. See Attributes for more information.

#### 3.4.3.26. `unsafe e`

¶

Elide safety checks and assertions (such as bounds checking) that
occur during execution of `e`

. This is useful if the compiler is
otherwise unable to avoid bounds checks (e.g. when using indirect
indexes), but you really do not want them there. Make very sure that
the code is correct; eliding such checks can lead to memory
corruption.

This construct is deprecated. Use the `#[unsafe]`

attribute instead.

#### 3.4.3.27. `assert cond e`

¶

Terminate execution with an error if `cond`

evaluates to false,
otherwise produce the result of evaluating `e`

. Unless `e`

produces a value that is used subsequently (it can just be a
variable), dead code elimination may remove the assertion.

#### 3.4.3.28. `a with [i] = e`

¶

Return `a`

, but with the element at position `i`

changed to
contain the result of evaluating `e`

. Consumes `a`

.

#### 3.4.3.29. `r with f = e`

¶

Return the record `r`

, but with field `f`

changed to have value
`e`

. The type of the field must remain unchanged. Type inference
is limited: `r`

must have a *completely known type* up to `f`

.
This sometimes requires extra type annotations to make the type of
`r`

known.

#### 3.4.3.30. `if c then a else b`

¶

If `c`

evaluates to `true`

, evaluate `a`

, else evaluate `b`

.

### 3.4.4. Binding Expressions¶

#### 3.4.4.1. `let pat = e in body`

¶

Evaluate `e`

and bind the result to the irrefutable pattern `pat`

(see Patterns) while evaluating `body`

. The `in`

keyword
is optional if `body`

is a `let`

expression.

#### 3.4.4.2. `let a[i] = v in body`

¶

Write `v`

to `a[i]`

and evaluate `body`

. The given index need
not be complete and can also be a slice, but in these cases, the value
of `v`

must be an array of the proper size. This notation is
Syntactic sugar for `let a = a with [i] = v in a`

.

#### 3.4.4.3. `let f params... = e in body`

¶

Bind `f`

to a function with the given parameters and definition
(`e`

) and evaluate `body`

. The function will be treated as
aliasing any free variables in `e`

. The function is not in scope of
itself, and hence cannot be recursive.

#### 3.4.4.4. `loop pat = initial for x in a do loopbody`

¶

Bind

`pat`

to the initial values given in`initial`

.For each element

`x`

in`a`

, evaluate`loopbody`

and rebind`pat`

to the result of the evaluation.Return the final value of

`pat`

.

The `= initial`

can be left out, in which case initial values for
the pattern are taken from equivalently named variables in the
environment. I.e., `loop (x) = ...`

is equivalent to ```
loop (x = x)
= ...
```

.

#### 3.4.4.5. `loop pat = initial for x < n do loopbody`

¶

Equivalent to `loop (pat = initial) for x in [0..1..<n] do loopbody`

.

#### 3.4.4.6. `loop pat = initial while cond do loopbody`

¶

Bind

`pat`

to the initial values given in`initial`

.If

`cond`

evaluates to true, bind`pat`

to the result of evaluating`loopbody`

, and repeat the step.Return the final value of

`pat`

.

#### 3.4.4.7. `match x case p1 -> e1 case p2 -> e2`

¶

Match the value produced by `x`

to each of the patterns in turn,
picking the first one that succeeds. The result of the corresponding
expression is the value of the entire `match`

expression. All the
expressions associated with a `case`

must have the same type (but
not necessarily match the type of `x`

). It is a type error if there
is not a `case`

for every possible value of `x`

- inexhaustive
pattern matching is not allowed.

### 3.4.5. Function Expressions¶

#### 3.4.5.1. `\x y z: t -> e`

¶

Produces an anonymous function taking parameters `x`

, `y`

, and
`z`

, returns type `t`

, and whose body is `e`

. Lambdas do not
permit type parameters; use a named function if you want a polymorphic
function.

#### 3.4.5.2. `(binop)`

¶

An *operator section* that is equivalent to `\x y -> x *binop* y`

.

#### 3.4.5.3. `(x binop)`

¶

An *operator section* that is equivalent to `\y -> x *binop* y`

.

#### 3.4.5.4. `(binop y)`

¶

An *operator section* that is equivalent to `\x -> x *binop* y`

.

#### 3.4.5.5. `(.a.b.c)`

¶

An *operator section* that is equivalent to `\x -> x.a.b.c`

.

#### 3.4.5.6. `(.[i,j])`

¶

An *operator section* that is equivalent to `\x -> x[i,j]`

.

## 3.5. Higher-order functions¶

At a high level, Futhark functions are values, and can be used as any
other value. However, to ensure that the compiler is able to compile
the higher-order functions efficiently via *defunctionalisation*,
certain type-driven restrictions exist on how functions can be used.
These also apply to any record or tuple containing a function (a
*functional type*):

Arrays of functions are not permitted.

A function cannot be returned from an

`if`

expression.A

`loop`

parameter cannot be a function.

Further, *type parameters* are divided into *non-lifted* (bound with
an apostrophe, e.g. `'t`

), *size-lifted* (`'~t`

), and *fully
lifted* (`'^t`

). Only fully lifted type parameters may be
instantiated with a functional type. Within a function, a lifted type
parameter is treated as a functional type.

See also In-place updates for details on how uniqueness types interact with higher-order functions.

## 3.6. Type Inference¶

Futhark supports Hindley-Milner-style type inference, so in many cases explicit type annotations can be left off. Record field projection cannot in isolation be fully inferred, and may need type annotations where their inputs are bound. The same goes when constructing sum types, as Futhark cannot assume that a given constructor only belongs to a single type. Further, unique types (see In-place updates) must be explicitly annotated.

Type inference processes top-level declared in top-down order, and the type of a top-level function must be completely inferred at its definition site. Specifically, if a top-level function uses overloaded arithmetic operators, the resolution of those overloads cannot be influenced by later uses of the function.

## 3.7. Size Types¶

Futhark supports a simple system of size-dependent types that statically verifies that the sizes of arrays passed to a function are compatible. The focus is on simplicity, not completeness.

Whenever a pattern occurs (in `let`

, `loop`

, and function
parameters), as well as in return types, *size annotations* may be
used to express invariants about the shapes of arrays that are
accepted or produced by the function. For example:

```
let f [n] (a: [n]i32) (b: [n]i32): [n]i32 =
map2 (+) a b
```

We use a *size parameter*, `[n]`

, to explicitly quantify sizes. The
`[n]`

parameter is not explicitly passed when calling `f`

.
Rather, its value is implicitly deduced from the arguments passed for
the value parameters. An array can contain *anonymous dimensions*,
e.g. `[]i32`

, for which the type checker will invent fresh size
parameters, which ensures that all arrays have a (symbolic) size.

A size annotation can also be an integer constant (with no suffix). Size parameters can be used as ordinary variables within the scope of the parameters. The type checker verifies that the program obeys any constraints imposed by size annotations.

*Size-dependent types* are supported, as the names of parameters can
be used in the return type of a function:

```
let replicate 't (n: i64) (x: t): [n]t = ...
```

An application `replicate 10 0`

will have type `[10]i32`

.

Whenever we write a type `[n]t`

, `n`

must already be a variable of
type `i64`

in scope (possibly by being bound as a size parameter).

### 3.7.1. Unknown sizes¶

Since sizes must be constants or variables, there are many cases where
the type checker cannot assign a precise size to the result of some
operation. For example, the type of `concat`

should conceptually be:

```
val concat [n] [m] 't : [n]t -> [m]t -> [n+m]t
```

But this is not presently allowed. Instead, the return type contains an anonymous size:

```
val concat [n] [m] 't : [n]t -> [m]t -> []t
```

When an application `concat xs ys`

is found, the result will be of
type `[k]t`

, where `k`

is a fresh *unknown* size variable that is
considered distinct from every other size in the program. It is often
necessary to perform a size coercion (see Size coercion) to
convert an unknown size to a known size.

Generally, unknown sizes are constructed whenever the true size cannot be expressed. The following lists all possible sources of unknown sizes.

#### 3.7.1.1. Size going out of scope¶

An unknown size is created when the proper size of an array refers to a name that has gone out of scope:

```
let c = a + b
in replicate c 0
```

The type of `replicate c 0`

is `[c]i32`

, but since `c`

is
locally bound, the type of the entire expression is `[k]i32`

for
some fresh `k`

.

#### 3.7.1.2. Compound expression passed as function argument¶

Intuitively, the type of `replicate (x+y) 0`

should be `[x+y]i32`

,
but since sizes must be names or constants, this is not expressible.
Therefore an unknown size variable is created and the size of the
expression becomes `[k]i32`

.

#### 3.7.1.3. Compound expression used as range bound¶

While a simple range expression such as `0..<n`

can be assigned type
`[n]i32`

, a range expression `0..<(n+1)`

will give produce an
unknown size.

#### 3.7.1.4. Complex slicing¶

Most complex array slicing, such as `xs[a:b]`

, will have an unknown
size. Exceptions are listed in the reference for slice
expressions.

#### 3.7.1.5. Complex ranges¶

Most complex ranges, such as `a..<b`

, will have an unknown size.
Exceptions exist for general ranges and “upto”
ranges.

#### 3.7.1.6. Anonymous size in function return type¶

Whenever the result of a function application would have an anonymous size, that size is replaced with a fresh unknown size variable.

For example, `filter`

has the following type:

```
val filter [n] 'a : (p: a -> bool) -> (as: [n]a) -> []a
```

Naively, an application `filter f xs`

seems like it would have type
`[]a`

, but a fresh unknown size `k`

will be created and the actual
type will be `[k]a`

.

#### 3.7.1.7. Branches of `if`

return arrays of different sizes¶

When an `if`

(or `match`

) expression has branches that returns
array of different sizes, the differing sizes will be replaced with
fresh unknown sizes. For example:

```
if b then [[1,2], [3,4]]
else [[5,6]]
```

This expression will have type `[k][2]i32`

, for some fresh `k`

.

**Important:** The check whether the sizes differ is done when first
encountering the `if`

or `match`

during type checking. At this
point, the type checker may not realise that the two sizes are
actually equal, even though constraints later in the function force
them to be. This can always be resolved by adding type annotations.

#### 3.7.1.8. An array produced by a loop does not have a known size¶

If the size of some loop parameter is not maintained across a loop iteration, the final result of the loop will contain unknown sizes. For example:

```
loop xs = [1] for i < n do xs ++ xs
```

Similar to conditionals, the type checker may sometimes be too cautious in assuming that some size may change during the loop. Adding type annotations to the loop parameter can be used to resolve this.

### 3.7.2. Size coercion¶

Size coercion, written with `:>`

, can be used to perform a
runtime-checked coercion of one size to another. Since size
annotations can refer only to variables and constants, this is
necessary when writing more complicated size functions:

```
let concat_to 'a (m: i32) (a: []a) (b: []a) : [m]a =
a ++ b :> [m]a
```

Only expression-level type annotations give rise to run-time checks. Despite their similar syntax, parameter and return type annotations must be valid at compile-time, or type checking will fail.

### 3.7.3. Causality restriction¶

Conceptually, size parameters are assigned their value by reading the sizes of concrete values passed along as parameters. This means that any size parameter must be used as the size of some parameter. This is an error:

```
let f [n] (x: i32) = n
```

The following is not an error:

```
let f [n] (g: [n]i32 -> [n]i32) = ...
```

However, using this function comes with a constraint: whenever an
application `f x`

occurs, the value of the size parameter must be
inferable. Specifically, this value must have been used as the size
of an array *before* the `f x`

application is encountered. The
notion of “before” is subtle, as there is no evaluation ordering of a
Futhark expression, *except* that a `let`

-binding is always
evaluated before its body, the argument to a function is always
evaluated before the function itself, and the left operand to an
operator is evaluated before the right.

The causality restriction only occurs when a function has size
parameters whose first use is *not* as a concrete array size. For
example, it does not apply to uses of the following function:

```
let f [n] (arr: [n]i32) (g: [n]i32 -> [n]i32) = ...
```

This is because the proper value of `n`

can be read directly from
the actual size of the array.

### 3.7.4. Empty array literals¶

Just as with size-polymorphic functions, when constructing an empty
array, we must know the exact size of the (missing) elements. For
example, in the following program we are forcing the elements of `a`

to be the same as the elements of `b`

, but the size of the elements
of `b`

are not known at the time `a`

is constructed:

```
let main (b: bool) (xs: []i32) =
let a = [] : [][]i32
let b = [filter (>0) xs]
in a[0] == b[0]
```

The result is a type error.

### 3.7.5. Sum types¶

When constructing a value of a sum type, the compiler must still be
able to determine the size of the constructors that are *not* used.
This is illegal:

```
type sum = #foo ([]i32) | #bar ([]i32)
let main (xs: *[]i32) =
let v : sum = #foo xs
in xs
```

### 3.7.6. Modules¶

When matching a module with a module type (see Module System),
a non-lifted abstract type (i.e. one that is declared with `type`

rather than `type^`

) may not be implemented by a type abbreviation
that contains any anonymous sizes. This is to ensure that if we have
the following:

```
module m : { type t } = ...
```

Then we can construct an array of values of type `m.t`

without
worrying about constructing an irregular array.

### 3.7.7. Higher-order functions¶

When a higher-order function takes a functional argument whose return
type is a non-lifted type parameter, any instantiation of that type
parameter must have a non-anonymous size. If the return type is a
lifted type parameter, then the instantiation may contain anonymous
sizes. This is why the type of `map`

guarantees regular arrays:

```
val map [n] 'a 'b : (a -> b) -> [n]a -> [n]b
```

The type parameter `b`

can only be replaced with a type that has
non-anonymous sizes, which means they must be the same for every
application of the function. In contrast, this is the type of the
pipeline operator:

```
val (|>) '^a -> '^b : a -> (a -> b) -> b
```

The provided function can return something with an anonymous size
(such as `filter`

).

#### 3.7.7.1. A function whose return type has an unknown size¶

If a function (named or anonymous) is inferred to have a return type
that contains an unknown size variable created *within* the function
body, that size variable will be replaced with an anonymous size. In
most cases this is not important, but it means that an expression like
the following is ill-typed:

```
map (\xs -> iota (length xs)) (xss : [n][m]i32)
```

This is because the `(length xs)`

expression gives rise to some
fresh size `k`

. The lambda is then assigned the type ```
[n]t ->
[k]i32
```

, which is immediately turned into `[n]t -> []i32`

because
`k`

was generated inside its body. A function of this type cannot
be passed to `map`

, as explained before. The solution is to bind
`length`

to a name *before* the lambda.

## 3.8. In-place Updates¶

In-place updates do not provide observable side effects, but they do provide a way to efficiently update an array in-place, with the guarantee that the cost is proportional to the size of the value(s) being written, not the size of the full array.

The `a with [i] = v`

language construct, and derived forms,
performs an in-place update. The compiler verifies that the original
array (`a`

) is not used on any execution path following the in-place
update. This involves also checking that no *alias* of `a`

is used.
Generally, most language constructs produce new arrays, but some
(slicing) create arrays that alias their input arrays.

When defining a function parameter or return type, we can mark it as
*unique* by prefixing it with an asterisk. For example:

```
let modify (a: *[]i32) (i: i32) (x: i32): *[]i32 =
a with [i] = a[i] + x
```

For bulk in-place updates with multiple values, use the `scatter`

function in the basis library. In the parameter declaration ```
a:
*[i32]
```

, the asterisk means that the function `modify`

has been
given “ownership” of the array `a`

, meaning that any caller of
`modify`

will never reference array `a`

after the call again.
This allows the `with`

expression to perform an in-place update.

After a call `modify a i x`

, neither `a`

or any variable that
*aliases* `a`

may be used on any following execution path.

### 3.8.1. Alias Analysis¶

The rules used by the Futhark compiler to determine aliasing are
intuitive in the intra-procedural case. Aliases are associated with
entire arrays. Aliases of a record are tuple are tracked for each
element, not for the record or tuple itself. Most constructs produce
fresh arrays, with no aliases. The main exceptions are `if`

,
`loop`

, function calls, and variable literals.

After a binding

`let a = b`

, that simply assigns a new name to an existing variable, the variable`a`

aliases`b`

. Similarly for record projections and patterns.The result of an

`if`

aliases the union of the aliases of the components.The result of a

`loop`

aliases the initial values, as well as any aliases that the merge parameters may assume at the end of an iteration, computed to a fixed point.The aliases of a value returned from a function is the most interesting case, and depends on whether the return value is declared

*unique*(with an asterisk`*`

) or not. If it is declared unique, then it has no aliases. Otherwise, it aliases all arguments passed for*non-unique*parameters.

### 3.8.2. In-place Updates and Higher-Order Functions¶

Uniqueness typing generally interacts poorly with higher-order functions. The issue is that we cannot control how many times a function argument is applied, or to what, so it is not safe to pass a function that consumes its argument. The following two conservative rules govern the interaction between uniqueness types and higher-order functions:

In the expression

`let p = e1 in ...`

, if*any*in-place update takes place in the expression`e1`

, the value bound by`p`

must not be or contain a function.A function that consumes one of its arguments may not be passed as a higher-order argument to another function.

## 3.9. Module System¶

mod_bind::= "module"`id`

`mod_param`

* "=" [":" mod_type_exp] "="`mod_exp`

mod_param::= "("`id`

":"`mod_type_exp`

")"mod_type_bind::= "module" "type"`id`

`type_param`

* "="`mod_type_exp`

Futhark supports an ML-style higher-order module system. *Modules*
can contain types, functions, and other modules and module types.
*Module types* are used to classify the contents of modules, and
*parametric modules* are used to abstract over modules (essentially
module-level functions). In Standard ML, modules, module types and
parametric modules are called structs, signatures, and functors,
respectively. Module names exist in the same name space as values,
but module types are their own name space.

Named modules are declared as:

```
module name = ...
```

A named module type is defined as:

```
module type name = ...
```

Where a module expression can be the name of another module, an application of a parametric module, or a sequence of declarations enclosed in curly braces:

```
module Vec3 = {
type t = ( f32 , f32 , f32 )
let add(a: t) (b: t): t =
let (a1, a2, a3) = a in
let (b1, b2, b3) = b in
(a1 + b1, a2 + b2 , a3 + b3)
}
module AlsoVec3 = Vec3
```

Functions and types within modules can be accessed using dot notation:

```
type vector = Vec3.t
let double(v: vector): vector = Vec3.add v v
```

We can also use `open Vec3`

to bring the names defined by `Vec3`

into the current scope. Multiple modules can be opened simultaneously
by separating their names with spaces. In case several modules define
the same names, the ones mentioned last take precedence. The first
argument to `open`

may be a full module expression.

Named module types are defined as:

```
module type ModuleTypeName = ...
```

A module type expression can be the name of another module type, or a
sequence of *specifications*, or *specs*, enclosed in curly braces. A
spec can be a *value spec*, indicating the presence of a function or
value, an *abstract type spec*, or a *type abbreviation spec*. For
example:

```
module type Addable = {
type t -- abstract type spec
type two_ts = (t,t) -- type abbreviation spec
val add: t -> t -> t -- value spec
}
```

This module type specifies the presence of an *abstract type* `t`

,
as well as a function operating on values of type `t`

. We can use
*module type ascription* to restrict a module to what is exposed by
some module type:

```
module AbstractVec = Vec3 : Addable
```

The definition of `AbstractVec.t`

is now hidden. In fact, with this
module type, we can neither construct values of type `AbstractVec.T`

or convert them to anything else, making this a rather useless use of
abstraction. As a derived form, we can write `module M: S = e`

to
mean `module M = e : S`

.

In a value spec, sizes in types on the left-hand side of a function arrow must not be anonymous. For example, this is forbidden:

```
val sum: []t -> t
```

Instead write:

```
val sum [n]: [n]t -> t
```

But this is allowed, because the empty size is not to the left of a function arrow:

```
val evens [n]: [n]i32 -> []i32
```

Parametric modules allow us to write definitions that abstract over modules. For example:

```
module Times = \(M: Addable) -> {
let times (x: M.t) (k: i32): M.t =
loop x' = x for i < k do
M.add x' x
}
```

We can instantiate `Times`

with any module that fulfils the module
type `Addable`

and get back a module that defines a function
`times`

:

```
module Vec3Times = Times Vec3
```

Now `Vec3Times.times`

is a function of type ```
Vec3.t -> int ->
Vec3.t
```

. As a derived form, we can write `module M p = e`

to mean
`module M = \p -> e`

.

### 3.9.1. Module Expressions¶

mod_exp::=`qualid`

|`mod_exp`

":"`mod_type_exp`

| "\" "("`id`

":"`mod_type_exp`

")" [":"`mod_type_exp`

] "->"`mod_exp`

|`mod_exp`

`mod_exp`

| "("`mod_exp`

")" | "{"`dec`

* "}" | "import"`stringlit`

A module expression produces a module. Modules are collections of bindings produced by declarations (dec). In particular, a module may contain other modules or module types.

#### 3.9.1.1. `qualid`

¶

Evaluates to the module of the given name.

#### 3.9.1.2. `(mod_exp)`

¶

Evaluates to `mod_exp`

.

#### 3.9.1.3. `mod_exp : mod_type_exp`

¶

*Module ascription* evaluates the module expression and the module
type expression, verifies that the module implements the module type,
then returns a module that exposes only the functionality described by
the module type. This is how internal details of a module can be
hidden.

#### 3.9.1.4. `\(p: mt1): mt2 -> e`

¶

Constructs a *parametric module* (a function at the module level) that
accepts a parameter of module type `mt1`

and returns a module of
type `mt2`

. The latter is optional, but the parameter type is not.

#### 3.9.1.5. `e1 e2`

¶

Apply the parametric module `m1`

to the module `m2`

.

#### 3.9.1.6. `{ decs }`

¶

Returns a module that contains the given definitions. The resulting
module defines any name defined by any declaration that is not
`local`

, *in particular* including names made available via
`open`

.

#### 3.9.1.7. `import "foo"`

¶

Returns a module that contains the definitions of the file `"foo"`

relative to the current file. See Referring to Other Files.

### 3.9.2. Module Type Expressions¶

mod_type_exp::=`qualid`

| "{"`spec`

* "}" |`mod_type_exp`

"with"`qualid`

`type_param`

* "="`type`

| "("`mod_type_exp`

")" | "("`id`

":"`mod_type_exp`

")" "->"`mod_type_exp`

|`mod_type_exp`

"->"`mod_type_exp`

spec::= "val"`id`

`type_param`

* ":"`spec_type`

| "val"`binop`

`type_param`

* ":"`spec_type`

| "type" ["^"]`id`

`type_param`

* "="`type`

| "type" ["^"]`id`

`type_param`

* | "module"`id`

":"`mod_type_exp`

| "include"`mod_type_exp`

| "#[" attr "]" specspec_type::=`type`

|`type`

"->"`spec_type`

Module types classify modules, with the only (unimportant) difference
in expressivity being that modules can contain module types, but
module types cannot specify that a module must contain a specific
module type. They can specify of course that a module contains a
*submodule* of a specific module type.

## 3.10. Referring to Other Files¶

You can refer to external files in a Futhark file like this:

```
import "file"
```

The above will include all non-`local`

top-level definitions from
`file.fut`

is and make them available in the current file (but
will not export them). The `.fut`

extension is implied.

You can also include files from subdirectories:

```
import "path/to/a/file"
```

The above will include the file `path/to/a/file.fut`

relative to the
including file.

Qualified imports are also possible, where a module is created for the file:

```
module M = import "file"
```

In fact, a plain `import "file"`

is equivalent to:

```
local open import "file"
```

## 3.11. Attributes¶

attr::=`id`

|`id`

"(" [`attr`

(","`attr`

)*] ")"

An expression, declaration, or module type spec can be prefixed with
an attribute, written as `#[attr]`

. This may affect how it is
treated by the compiler or other tools. In no case will attributes
affect or change the *semantics* of a program, but it may affect how
well it compiles and runs (or in some cases, whether it compiles or
runs at all). Unknown attributes are silently ignored. Most have no
effect in the interpreter. An attribute can be either an *atom*,
written as just an identifier, or *compound*, consisting of an
identifier and a comma-separated sequence of attributes. The latter
is used for grouping and encoding of more complex information.

### 3.11.1. Expression attributes¶

Many expression attributes affect second-order array combinators
(*SOACS*). These must be applied to a fully saturated function
application or they will have no effect. If two SOACs with
contradictory attributes are combined through fusion, it is
unspecified which attributes take precedence.

The following expression attributes are supported.

#### 3.11.1.1. `incremental_flattening(no_outer)`

¶

When using incremental flattening, do not generate the “only outer parallelism” version for the attributed SOACs.

#### 3.11.1.2. `incremental_flattening(no_intra)`

¶

When using incremental flattening, do not generate the “intra-group parallelism” version for the attributed SOACs.

#### 3.11.1.3. `incremental_flattening(only_intra)`

¶

When using incremental flattening, *only* generate the “intra-group
parallelism” version of the attributed SOACs. **Beware**: the
resulting program will fail to run if the inner parallelism does not
fit on the device.

#### 3.11.1.4. `incremental_flattening(only_inner)`

¶

When using incremental flattening, do not generate multiple versions for this SOAC, but do exploit inner parallelism (which may give rise to multiple versions at deeper levels).

#### 3.11.1.5. `noinline`

¶

Do not inline the attributed function application. If used within a
parallel construct (e.g. `map`

), this will likely prevent the GPU
backends from generating working code.

#### 3.11.1.6. `sequential`

¶

*Fully* sequentialise the attributed SOAC.

#### 3.11.1.7. `sequential_outer`

¶

Turn the outer parallelism in the attributed SOAC sequential, but preserve any inner parallelism.

#### 3.11.1.8. `sequential_inner`

¶

Exploit only outer parallelism in the attributed SOAC.

#### 3.11.1.9. `unroll`

¶

Fully unroll the attributed `loop`

. If the compiler cannot
determine the exact number of iterations (possibly after other
optimisations and simplifications have taken place), then this
attribute has no code generation effect, but instead results in a
warning. Be very careful with this attribute: it can massively
increase program size (possibly crashing the compiler) if the loop has
a huge number of iterations.

#### 3.11.1.10. `unsafe`

¶

Do not perform any dynamic safety checks (such as bound checks) during execution of the attributed expression.

#### 3.11.1.11. `warn(safety_checks)`

¶

Make the compiler issue a warning if the attributed expression (or its subexpressions) requires safety checks (such as bounds checking) at run-time. This is used for performance-critical code where you want to be told when the compiler is unable to statically verify the safety of all operations.

### 3.11.2. Declaration attributes¶

The following declaration attributes are supported.

#### 3.11.2.1. `noinline`

¶

Do not inline any calls to this function. If the function is then
used within a parallel construct (e.g. `map`

), this will likely
prevent the GPU backends from generating working code.

### 3.11.3. Spec attributes¶

No spec attributes are currently supported by the compiler itself, although they are syntactically permitted and may be used by other tools.