3. Language Overview¶
The Futhark programming language is a purely functional, call-by-value, mostly first-order language that permits bulk operations on arrays using second-order array combinators (SOACs).
The primary idea behind Futhark is to design a language that has enough expressive power to conveniently express complex programs, yet is also amenable to aggressive optimisation and parallelisation. The tension is that as the expressive power of a language grows, the difficulty of efficient compilation rises likewise. For example, Futhark supports nested parallelism, despite the complexities of efficiently mapping it to the flat parallelism supported by hardware, as many algorithms are awkward to write with just flat parallelism. On the other hand, we do not support non-regular arrays, as they complicate size analysis a great deal. The fact that Futhark is purely functional is intended to give an optimising compiler more leeway in rearranging the code and performing high-level optimisations. It is also the plan to eventually design a rigorous cost model for Futhark, although this work has not yet been completed.
3.1. Lexical Syntax¶
The syntax of Futhark is derived from Haskell and Standard ML,
although somewhat less flexible. Futhark is not whitespace-sensitive,
and indentation is only used for readability. An identifier starts
with a lowercase letter, followed by any number of letters, digits,
apostrophes and underscores. An identifier may also start with an
underscore, which is used to indicate that the name will not be used.
A structure identifier starts with an uppercase letter. Numeric,
string and character literals use the same notation as Haskell (which
is very similar to C), including all escape characters. Comments are
-- and span to end of line. Block comments are not
3.2. Simple Futhark¶
An Futhark program consists of a sequence of function definitions, of the following form:
let name params... : return_type = body
A function must declare both its return type and the types of all its parameters. All functions (except for inline anonymous functions; see below) are defined globally. Futhark does not use type inference. As a concrete example, here is the a parallel definition of the factorial function in Futhark:
let fact(n: i32): i32 = reduce (*) 1 (map (1+) (iota n))
Indentation has no syntactical significance in Futhark, but recommended for readability.
The syntax for tuple types is a comma-separated list of types or
values enclosed in parentheses, so
(i32, real) is a pair of an
integer and a floating-point number. Single-element and empty tuples
are not permitted. Array types are written as the element type
preceded by brackets, meaning that
i32 is a one-dimensional
array of integers, and
(i32, real) is a three-dimensional
array of tuples of integers and floats. An array value is written as
a sequence of elements enclosed by brackets:
[1, 2, 3] -- Array of type i32. [, , ] -- Array of type i32.
All arrays must be regular (often termed full). This means that, for example, all rows of a two-dimensional array must have the same number of elements:
[[1, 2], ] -- Compile-time error. [iota(1), iota(2)] -- A run-time error if reached.
The restriction to regular arrays simplifies compilation.
Arrays are indexed using the common row-major notation, e.g.,
i2, i3...]. An indexing is said to be full if the number of given
indices is equal to the dimensionality of the array.
let-expression can be used to refer to the result of a
let z = x + y in ...
Recall that Futhark is eagerly evaluated, so the right-hand side of
let is evaluated exactly once, at the time it is first
in keyword is optional when it precedes another
let. This means that instead of writing:
let a = 0 in let b = 1 in let c = 2 in a + b + c
we can write:
let a = 0 let b = 1 let c = 2 in a + b + c
in is still necessary.
if-then-else is the only branching construct in Futhark.
Pattern matching is supported in a limited way for taking apart
tuples, for example:
let sumpair((x, y): (i32, i32)): i32 = x + y
We can also add the type ascription on the tuple components:
let sumpair(x: i32, y: i32): i32 = x + y
Apart from pattern-matching, the components of a tuple can also be
accessed by using the tuple projection syntax:
indicates the component to extract (0-indexed):
let sumpair(p: (i32, i32)): i32 = #0 p + #1 p
In most cases, pattern matching is better style.
Function calls are written as the function name with the arguments juxtaposed. All function calls must be fully saturated - currying is only permitted in SOACs.
3.2.1. Sequential Loops¶
Futhark has a built-in syntax for expressing certain tail-recursive functions. Consider the following tail-recursive formulation of a function for computing the Fibonacci numbers:
let fib(n: i32): i32 = fibhelper(1,1,n) let fibhelper(x: i32, y: i32, n: i32): i32 = if n == 1 then x else fibhelper(y, x+y, n-1)
This is not valid Futhark, as Futhark does not presently allow
recursive functions. Instead, we can write this using the
let fib(n: i32): i32 = loop ((x, y) = (1,1)) = for i < n do (y, x+y) in x
The semantics of this is precisely as in the tail-recursive function formulation. In general, a loop:
loop (pat = initial) = for i < bound do loopbody in body
Has the following the semantics:
- Bind pat to the initial values given in initial.
- While i < bound, evaluate loopbody, rebinding pat to be the value returned by the body. At the end of each iteration, increment i by one.
- Evaluate body with pat bound to its final value.
loop expression is completely equivalent to a
call to its corresponding tail-recursive function.
For example, denoting by
t the type of
x, this loop:
loop (x = a) = for i < n do g(x) in body
has the semantics of a call to this tail-recursive function:
let f(i: i32, n: i32, x: t): t = if i >= n then x else f(i+1, n, g(x)) let x = f(i, n, a) in body
The purpose of
loop is partly to render some sequential
computations slightly more convenient, but primarily to express
certain very specific forms of recursive functions, specifically those
with a fixed iteration count. This property is used for analysis and
optimisation by the Futhark compiler.
Apart from the
i < n form, which loops from zero, Futhark also
v <= i < n form which starts at
v. We can also
invert the order of iteration by writing
n > i or
n > i >= v,
which loops down from the upper bound to the lower. Due to parser
limitations, most non-atomic expressions will have to be parenthesised
when used as the left-hand bound.
for-loops, Futhark also supports
These do not provide as much information to the compiler, but can be
used for convergence loops, where the number of iterations cannot be
predicted in advance. For example, the following program doubles a
given number until it exceeds a given threshold value:
let main(x: i32, bound: i32): i32 = loop (x) = while x < bound do x * 2 in x
In all respects other than termination criteria,
behave identically to
For brevity, the initial value expression can be elided, in which case an expression equivalent to the pattern is implied. This is easier to understand with an example. The loop:
let fib(n: i32): i32 = let x = 1 let y = 1 loop ((x, y) = (x, y)) = for i < n do (y, x+y) in x
can also be written:
let fib(n: i32): i32 = let x = 1 let y = 1 loop ((x, y)) = for i < n do (y, x+y) in x
This can sometimes make imperative code look more natural.
3.2.2. Shape Declarations¶
Optionally, the programmer may put shape declarations in the return type and parameter types of a function declaration. These can be used to express invariants about the shapes of arrays that are accepted or produced by the function, e.g:
let f (a: [#n]i32): [n]i32 = map (+1) a
The above declaration specifies a function that accepts an array
n elements and returns an array likewise containing
n elements. When prefixed with a
# character, a name is
freshly bound, whilst an unadorned name must be in scope. In the
example above, we do not use a
# in the return type, because we
wish to refer to the
n bound by the parameters. If we refer to
the same freshly bound variable in multiple parameters (see below),
each occurence must be prefixed with
#. A shape declaration can
also be an integer constant (with no suffix).
The same name can be used in several dimensions, or even in several parameters. This can be used to give a natural type to a function for computing dot products:
let dotProduct(a: [#n]i32, b: [#n]i32): i32 = reduce (+) 0 (map (*) a b)
Or matrix multiplication:
let matMult(x: [#n][#m]i32, y: [#m][#n]i32): [n][n]i32 = ...
The dimension names bound in a parameter shape declaration can be used as ordinary variables inside the scope of the parameter.
Shape declarations serve two main purposes:
- They document the shape assumptions of the function in an easily understandable form.
- More importantly, they help the compiler understand the invariants of the program, which it may otherwise have trouble figuring out.
Note that adding shape declarations is never unsafe - the compiler
still inserts dynamic checks, so if an incorrect declaration is made,
the result will merely be an abrubt but controlled termination as it
collides with reality. Shape declarations matter most when used for
the input parameters of the
main function and for the return type
of functions used to
3.2.3. In-Place Updates¶
In an array programming language, we tend to use bulk operations for
most array manipulation. However, sometimes it is useful to directly
replace some element. In a pure language, we cannot permit free
mutation, but we can permit the creation of a duplicate array, where
some elements have been changed. General modification of array
elements is done using the
let-with construct. In its most
general form, it looks as follows:
let dest = src with [indexes] <- (value) in body
dest bound to the value of
except that the element(s) at the position given by
on the new value
value. Due to parser limitations, the
value are not optional. The given indexes need
not be complete, but in that case,
value must be an array of the
proper size. As an example, here’s how we could replace the third row
n * 3 array:
let b = a with  <- ([1,2,3]) in b
As a convenience, whenever
src are the same, we can
let dest[indexes] = value in body
as a shortcut. Note that this has no special semantic meaning, but is simply a case of normal name shadowing.
For example, this loop implements the “imperative” version of matrix
multiplication of an
m * o with an
o * n matrix:
let matmult(a: [#m][#o]f32, b: [#o][#n]f32): [m][n]f32 = let res = replicate(m, replicate(n,0f32)) in loop (res) = for i < m do loop (res) = for j < n do loop (partsum = 0f32) = for k < o do partsum + a[i,k] * b[k,j] let res[i,j] = partsum in res in res in res
With the naive implementation based on copying the source array,
let-with expression would require memory
proportional to the entire source array, rather than proportional to
the slice we are changing. This is not ideal. Therefore, the
let-with construct has some unusual restrictions to permit
in-place modification of the
src array, as described in
Uniqueness Types. Simply put, we track that
src is never used
again. The consequence is that we can guarantee that the execution of
let-with expression does not involve any copying of the source
array in order to create the newly bound array, and therefore the time
required for the update is proportional to the section of the array we
are updating, not the entire array. We can think of this as similar
to array modification in an imperative language.
The language presented in the previous section is in some sense
“sufficient”, in that it is Turing-complete, and can express
imperative-style loops in a natural way with
while-loops. However, Futhark is not intended to be used in this
way - bulk operations on arrays should be expressed via one of the
second-order array combinators (SOACs) shown below, as this
maximises the amount of parallelism that the compiler is able to take
e ::= "map"
A lambda can be an anonymous function, the name of a function (with optional curried arguments), or an operator (possibly with one operand curried):
lambda ::= "(" \
Parameter- and return type ascriptions are optional in anonymous functions. The semantics of the SOACs is identical to the similarly-named higher-order functions found in many functional languages. For specifics, see Language Reference.
scan SOAC performs an inclusive prefix scan, and returns an
array of the same outer size as the original array. The functions
scan must be binary associative operators,
and the value given as the initial value of the accumulator must be
the neutral element for the function. These properties are not
checked by the Futhark compiler, and are the responsibility of the
3.4. Uniqueness Types¶
While Futhark is uncompromisingly a pure functional language, it may occasionally prove useful to express certain algorithms in an imperative style. Consider a function for computing the n first Fibonacci numbers:
let fib(n: i32): i32 = -- Create "empty" array. let arr = iota(n) in -- Fill array with Fibonacci numbers. loop (arr) = for i < n-2 do let arr[i+2] = arr[i] + arr[i+1] in arr in arr
If the array
arr is copied for each iteration of the loop, we
are going to put enormous pressure on memory, and spend a lot of time
moving around data, even though it is clear in this case that the
“old” value of
arr will never be used again. Precisely,
what should be an algorithm with complexity O(n) becomes (n^2)
due to copying the size n array (an O(n) operation) for each of
the n iterations of the loop.
To prevent this, we will want to update the array in-place,
that is, with a static guarantee that the operation will not require
any additional memory allocation, such as copying the entire array. With an
in-place modification, a
let-with can modify the array in
time proportional to the slice being updated (O(1) in the case of
the Fibonacci function), rather than time proportional to the size of
the final array, as would the case if we performed a full copy. In order to
perform the update without violating referential transparency, we need
to know that no other references to the array exists, or at least that
such references will not be used on any execution path following the
In Futhark, this is done through a type system feature called
uniqueness types, similar to, although simpler, than the uniqueness
types of Clean. Alongside a (relatively) simple aliasing analysis in
the type checker, this is sufficient to determine at compile time
whether an in-place modification is safe, and signal a compile time
let-with is used in way where safety cannot be
The simplest way to introduce uniqueness types is through examples. To that end, let us consider the following function definition:
let modify(a: *i32, i: i32, x: i32): *i32 = let a[i] = a[i] + x in a
The function call
a, but where the
element at index
i has been increased by
x. Note the
asterisks in the parameter declaration
*i32 a. This means that
modify has been given “ownership” of the array
meaning that the caller of
modify will never reference array
the call. As a consequence,
modify can change the element at index
i without first copying the array, i.e.
modify is free to do
an in-place modification. Furthermore, the return value of
is also unique - this means that the result of the call to
does not share elements with any other visible variables.
Let us consider a call to
modify, which might look as
let b = modify(a, i, x) in ..
Under which circumstances is this call valid? Two things must hold:
- The type of
*i32, of course.
aor any variable that aliases
amay be used on any execution path following the call to
In general, when a value is passed as a unique-typed argument in a function call, we consider that value to be consumed, and neither it nor any of its aliases can be used again. Otherwise, we would break the contract that gives the function liberty to manipulate the argument however it wants. Note that it is the type in the argument declaration that must be unique - it is permissible to pass a unique-typed variable as a non-unique argument (that is, a unique type is a subtype of the corresponding nonunique type).
A variable v aliases a if they may share some elements,
i.e. overlap in memory. As the most trivial case, after evaluating
let b = a, the variable
b will alias
a. As another example, if we extract a row from a
two-dimensional array, the row will alias its source:
let b = a in ... -- b is aliased to a (assuming a is not one-dimensional)
In Sharing Analysis below, we will cover sharing and sharing analysis in greater detail.
Let us consider the definition of a function returning a unique array:
let f(a: i32): *i32 = body
Note that the argument,
a, is non-unique, and hence we cannot
modify it. There is another restriction as well:
a must not be
aliased to our return value, as the uniqueness contract requires us to
ensure that there are no other references to the unique return value.
This requirement would be violated if we permitted the return value in
a unique-returning function to alias its non-unique parameters.
To summarise: values are consumed by being the source in a ``let-with``, or by being passed as a unique parameter in a function call. We can crystallise valid usage in the form of three principal rules:
Uniqueness Rule 1
When a value is passed in the place of a unique parameter in a function call, or used as the source in a
let-withexpression, neither that value, nor any value that aliases it, may be used on any execution path following the function call. An example violation:let b = a let b[i] = 2 in f(b,a) -- Error: a used after being source in a let-with
Uniqueness Rule 2
If a function definition is declared to return a unique value, the return value (that is, the result of the body of the function) must not share memory with any non-unique arguments to the function. As a consequence, at the time of execution, the result of a call to the function is the only reference to that value. An example violation:let broken(a: i32, i: i32): *i32 = a[i] -- Return value aliased with 'a'.
Uniqueness Rule 3If a function call yields a unique return value, the caller has exclusive access to that value. At the point the call returns, the return value may not share memory with any variable used in any execution path following the function call. This rule is particularly subtle, but can be considered a rephrasing of Uniqueness Rule 2 from the “calling side”.
It is worth emphasising that everything in this chapter is employed as part of a static analysis. All violations of the uniqueness rules will be discovered at compile time during type-checking, thus leaving the code generator and runtime system at liberty to exploit them for low-level optimisation.