# 8. Compiler Error Index¶

Elaboration on type errors produced by the compiler. Many error messages contain links to the sections below.

## 8.1. Uniqueness errors¶

### 8.1.1. “Using *x*, but this was consumed at *y*.”¶

A core principle of uniqueness typing (see In-place Updates) is that after a variable is “consumed”, it must not be used again. For example, this is invalid, and will result in the error above:

```
let y = x with [0] = 0
in x
```

Several operations can *consume* a variable: array update expressions,
calling a function with unique-typed parameters, or passing it as the
initial value of a unique-typed loop parameter. When a variable is
consumed, its *aliases* are also considered consumed. Aliasing is the
possibility of two variables occupying the same memory at run-time.
For example, this will fail as above, because `y`

and `x`

are
aliased:

```
let y = x
let z = y with [0] = 0
in x
```

We can always break aliasing by using a `copy`

expression:

```
let y = copy x
let z = y with [0] = 0
in x
```

### 8.1.2. “Would consume *x*, which is not consumable”¶

This error message occurs for programs that try to perform a consumption (such as an in-place update) on variables that are not consumable. For example, it would occur for the following program:

```
def f (a: []i32) =
let a[0] = a[0]+1
in a
```

Only arrays with a a *unique array type* can be consumed. Such a type
is written by prefixing the array type with an asterisk. The program
could be fixed by writing it like this:

```
def f (a: *[]i32) =
let a[0] = a[0]+1
in a
```

Note that this places extra obligations on the caller of the `f`

function, since it now *consumes* its argument. See
In-place Updates for the full details.

You can always obtain a unique copy of an array by using
`copy`

:

```
def f (a: []i32) =
let a = copy a
let a[0] = a[0]+1
in a
```

But note that in most cases (although not all), this subverts the purpose of using in-place updates in the first place.

### 8.1.3. “Unique-typed return value of *x* is aliased to *y*, which is not consumable”¶

This can be caused by a function like this:

```
def f (xs: []i32) : *[]i32 = xs
```

We are saying that `f`

returns a *unique* array - meaning it has no
aliases - but at the same time, it aliases the parameter *xs*, which
is not marked as being unique (see In-place Updates). This
violates one of the core guarantees provided by uniqueness types,
namely that a unique return value does not alias any value that might
be used in the future. Imagine if this was permitted, and we had a
program that used `f`

:

```
let b = f a
let b[0] = x
...
```

The update of `b`

is fine, but if `b`

was allowed to alias `a`

(hence occupying the same memory), then we would be modifying `a`

as
well, which is a violation of referential transparency.

As with most uniqueness errors, it can be fixed by using `copy xs`

to break the aliasing. We can also change the type of `f`

to take a
unique array as input:

```
def f (xs: *[]i32) : *[]i32 = xs
```

This makes `xs`

“consumable”, in the sense used by the error message.

### 8.1.4. “A unique-typed component of the return value of *x* is aliased to some other component”¶

Caused by programs like the following:

```
def main (xs: *[]i32) : (*[]i32, *[]i32) = (xs, xs)
```

While we are allowed to “consume” `xs`

, as it is a unique parameter,
this function is trying to return two unique values that alias each
other. This violates one of the core guarantees provided by
uniqueness types, namely that a unique return value does not alias any
value that might be used in the future (see In-place Updates) -
and in this case, the two values alias each other. We can fix this by
inserting copies to break the aliasing:

```
def main (xs: *[]i32) : (*[]i32, *[]i32) = (xs, copy xs)
```

### 8.1.5. “Consuming parameter passed non-unique argument”¶

Caused by programs like the following:

```
def update (xs: *[]i32) = xs with [0] = 0
def f (ys: []i32) = update ys
```

The update `function`

*consumes* its `xs`

argument to perform an
in-place update, as denoted by the asterisk
before the type. However, the `f`

function tries to pass an array
that it is not allowed to consume (no asterisk before the type).

One solution is to change the type of `f`

so that it also consumes
its input, which allows it to pass it on to `update`

:

```
def f (ys: *[]i32) = update ys
```

Another solution to `copy`

the array that we pass to `update`

:

```
def f (ys: []i32) = update (copy ys)
```

### 8.1.6. “Non-consuming higher-order parameter passed consuming argument.”¶

This error occurs when we have a higher-order function that expects a
function that does *not* consume its arguments, and we pass it one
that does:

```
def apply 'a 'b (f: a -> b) (x: a) = f x
def consume (xs: *[]i32) = xs with [0] = 0
def f (arr: *[]i32) = apply consume arr
```

We can fix this by changing `consume`

so that it does not have to
consume its argument, by adding a `copy`

:

```
def consume (xs: []i32) = copy xs with [0] = 0
```

Or we can create a variant of `apply`

that accepts a consuming
function:

```
def apply 'a 'b (f: *a -> b) (x: *a) = f x
```

### 8.1.7. “Function result aliases the free variable *x*”¶

Caused by definitions such as the following:

```
def x = [1,2,3]
def f () = x
```

To simplify the tracking of aliases, the Futhark type system requires
that the result of a function may only alias the function parameters,
not any free variables. Use `copy`

to fix this:

```
def f () = copy x
```

### 8.1.8. “Parameter *x* refers to size *y* which will not be accessible to the caller¶

This happens when the size of an array parameter depends on a name that cannot be expressed in the function type:

```
def f (x: i64, y: i64) (A: [x]bool) = true
```

Intuitively, this function might have the following type:

```
val f : (x: i64, y: i64) -> [x]bool -> bool
```

But this is not currently a valid Futhark type. In a function type,
each parameter can be named *as a whole*, but it cannot be taken apart
in a pattern. In this case, we could fix it by splitting the tuple
parameter into two separate parameters:

```
def f (x: i64) (y: i64) (A: [x]bool) = true
```

This gives the following type:

```
val f : (x: i64) -> (y: i64) -> [x]bool -> bool
```

Another workaround is to loosen the static safety, and use a size coercion to give A its expected size:

```
def f (x: i64, y: i64) (A_unsized: []bool) =
let A = A_unsized :> [x]bool
in true
```

This will produce a function with the following type:

```
val f [d] : (i64, i64) -> [d]bool -> bool
```

This does however lose the constraint that the size of the array must match one of the elements of the tuple, which means the program may fail at run-time.

The error is not always due to an explicit type annotation. It might also be due to size inference:

```
def f (x: i64, y: i64) (A: []bool) = zip A (iota x)
```

Here the type rules force `A`

to have size `x`

, leading to a
problematic type. It can be fixed using the techniques above.

## 8.2. Size errors¶

### 8.2.1. “Size *x* unused in pattern.”¶

Caused by expressions like this:

```
def [n] (y: i32) = x
```

And functions like this:

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

Since `n`

is not the size of anything, it cannot be assigned a value
at runtime. Hence this program is rejected.

### 8.2.2. “Causality check”¶

Causality check errors occur when the program is written in such a way that a size is needed before it is actually computed. See Causality restriction for the full rules. Contrived example:

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

Here the inner size of the array `a`

must be the same as the inner
size of `b`

, but the inner size of `b`

depends on a `filter`

operation that is executed after `a`

is constructed.

There are various ways to fix causality errors. In the above case, we
could merely change the order of statements, such that `b`

is bound
first, meaning that the size is available by the time `a`

is bound.
In many other cases, we can lift out the “size-producing” expressions
into a separate `let`

-binding preceding the problematic expressions.

### 8.2.3. “Unknowable size *x* in parameter of *y*”¶

This error occurs when you define a function that can never be applied, as it requires an input of a specific size, and that size is not known. Somewhat contrived example:

```
def f (x: bool) =
let n = if x then 10 else 20
in \(y: [n]bool) -> ...
```

The above constructs a function that accepts an array of size 10 or
20, based on the value of `x`

argument. But the type of `f true`

by itself would be `?[n].[n]bool -> bool`

, where the `n`

is
unknown. There is no way to construct an array of the right size, so
the type checker rejects this program. (In a fully dependently typed
language, the type would have been `[10]bool -> bool`

, but Futhark
does not do any type-level computation.)

In most cases, this error means you have done something you didn’t actually mean to. However, in the case that that the above really is what you intend, the workaround is to make the function fully polymorphic, and then perform a size coercion to the desired size inside the function body itself:

```
def f (x: bool) =
let n = if x then 10 else 20
in \(y_any: []bool) ->
let y = y_any :> [n]bool
in true
```

This requires a check at run-time, but it is the only way to accomplish this in Futhark.

### 8.2.4. “Existential size would appear in function parameter of return type”¶

This occurs most commonly when we use function composition with one or
more functions that return an *existential size*. Example:

```
filter (>0) >-> length
```

The `filter`

function has this type:

```
val filter [n] 't : (t -> bool) -> [n]t -> ?[m].[m]t
```

That is, `filter`

returns an array whose size is not known until the
function actually returns. The `length`

function has this type:

```
val length [n] 't : [n]t -> i64
```

Whenever `length`

occurs (as in the composition above), the type
checker must *instantiate* the `[n]`

with the concrete symbolic size
of its input array. But in the composition, that size does not
actually exist until `filter`

has been run. For that matter, the
type checker does not know what `>->`

does, and for all it knows it
may actually apply `filter`

many times to different arrays, yielding
different sizes. This makes it impossible to uniquely instantiate the
type of `length`

, and therefore the program is rejected.

The common workaround is to use *pipelining* instead of composition
whenever we use functions with existential return types:

```
xs |> filter (>0) |> length
```

This works because `|>`

is left-associative, and hence the ```
xs |>
filter (>0)
```

part will be fully evaluated to a concrete array before
`length`

is reached.

We can of course also write it as `length (filter (>0) xs)`

, with no
use of either pipelining or composition.

### 8.2.5. “Existential size *n* not used as array size”¶

This error occurs for type expressions that use explicit existential quantification in an incorrect way, such as the following examples:

```
?[n].bool
?[n].bool -> [n]bool
```

When we use existential quantification, we are required to use the
size within its scope, *and* it must not exclusively be used to the
right of function arrow.

To understand the motivation behind this rule, consider that when we
use an existential quantifier we are saying that there is *some size*,
it just cannot be known statically, but must be read from some value
(i.e. array) at runtime. In the first example above, the existential
size `n`

is not used at all, so the actual value cannot be
determined at runtime. In the second example, while an array
`[n]bool`

does exist, it is part of a function type, and at runtime
functions are black boxes and don’t “carry” the size of their
parameter or result types.

The workaround is to actually use the existential size. This can be
as simple as adding a *witness array* of type `[n]()`

:

```
?[n].([n](),bool)
?[n].([n](), bool -> [n]bool)
```

Such an array will take up no space at runtime.

### 8.2.6. “Parameter *x* used as size would go out of scope.”¶

This error tends to happen because higher-order functions are used in a way that causes a size requirement to become impossible to constrait. Real programs that run into this issue are quite complex, but to illustrate the problem, consider the following contrived function:

```
def f (n: i64) (m: i64) (b: [n][m]bool) = b[0,0]
```

We have the following type:

```
val f : (n: i64) -> (m: i64) -> (b: [n][m]bool) -> bool
```

Now suppose we say:

```
def g = uncurry f
```

What should be the type of `g`

? Intuitively, something like this:

```
val g : (n: i64, m: i64) -> (b: [n][m]bool) -> bool
```

But this is *not* expressible in the Futhark type system - and even if
it were, it would not be easy to infer this in general, as it depends
on exactly what `uncurry`

does, which the type checker does not
know.

As a workaround, we can use explicit type annotation and size
coercions to give `g`

an acceptable type:

```
def g [a][b] (n,m) (b: [a][b]bool) = f n m (b :> [n][m]bool)
```

Another workaround, which is often the right one in cases not as
contrived as above, is to modify `f`

itself to produce a *witness*
of the constraint, in the form of an array of shape `[n][m]`

:

```
def f (n: i64) (m: i64) : ([n][m](), [n][m]bool -> bool) =
(replicate n (replicate m ()), \b -> b[0,0])
```

Then `uncurry f`

works just fine and has the following type:

```
(i64, i64) -> ?[n][m].([n][m](), [n][m]bool -> bool)
```

Programming with such *explicit size witnesses* is a fairly advanced
technique, but often necessary when writing advanced size-dependent
code.

### 8.2.7. “Ambiguous size *x*”¶

There are various sources for this error, but they all have the same ultimate cause: the type checker cannot figure out how some symbolic size name should be resolved to a concrete size. The simplest example, although contrived, is probably this:

```
let [n][m] (xss: [n][m]i64) = []
```

The type checker can infer that `n`

should be zero, but how can it
possibly figure out the shape of the (non-existent) rows of the
two-dimensional array? This can be fixed in many ways, but adding a
type ascription to the array is one of them: `[] : [0][2]i64`

.

Another common case arises when using holes. For an expression
`length ???`

, how would the type checker figure out the intended
size of the array that the hole represents? Again, this can be solved
with a type ascription: `length (??? : [10]bool)`

.

Finally, ambiguous sizes can also occur for functions that use size parameters only in “non-witnessing” position, meaning sizes that are not actually uses as sizes of real arrays. An example:

```
def f [n] (g: [n]i64 -> i64) : i64 = n
def main = f (\xs -> xs[0])
```

Note that `f`

is a higher order function, and that the size
parameter `n`

is only used in the type of the `g`

function.
Futhark’s value model is such that given a value of type ```
[n]i64 ->
i64
```

, we cannot extract an `n`

from it. Using a function such as
`f`

is only valid when `n`

can be inferred from the usage, which
is not the case here. Again, we can fix it by adding a type
ascription to disambiguate:

```
def main = f (\(xs:[1]i64) -> xs[0])
```

## 8.3. Module errors¶

### 8.3.1. “Entry points may not be declared inside modules.”¶

This occurs when the program uses the `entry`

keyword inside a
module:

```
module m = {
entry f x = x + 1
}
```

Entry points can only be declared at the top level of a file. When we wish to make a function from inside a module available as an entry point, we must define a wrapper function:

```
module m = {
def f x = x + 1
}
entry f = m.f
```

### 8.3.2. “Module *x* is a parametric module¶

A parametric module is a module-level function:

```
module PM (P: {val x : i64}) = {
def y = x + 2
}
```

If we directly try to access the component of `PM`

, as `PM.y`

, we
will get an error. To use `PM`

we must first apply it to a module
of the expected type:

```
module M = PM { val x = 2 : i64 }
```

Now we can say `M.y`

. See Modules for more.

## 8.4. Other errors¶

### 8.4.1. “Literal out of bounds”¶

This occurs for overloaded constants such as `1234`

that are
inferred by context to have a type that is too narrow for their value.
Example:

```
257 : u8
```

It is not an error to have a *non-overloaded* numeric constant whose
value is too large for its type. The following is perfectly
cromulent:

```
257u8
```

In such cases, the behaviour is overflow (so this is equivalent to
`1u8`

).

### 8.4.2. “Type is ambiguous”¶

There are various cases where the type checker is unable to infer the full type of something. For example:

```
def f r = r.x
```

We know that `r`

must be a record with a field called `x`

, but
maybe the record could also have other fields as well. Instead of
assuming a perhaps too narrow type, the type checker signals an error.
The solution is always to add a type annotation in one or more places
to disambiguate the type:

```
def f (r: {x:bool, y:i32}) = r.x
```

Usually the best spot to add such an annotation is on a function parameter, as above. But for ambiguous sum types, we often have to put it on the return type. Consider:

```
def f (x: bool) = #some x
```

The type of this function is ambiguous, because the type checker must
know what other possible contructors (apart from `#some`

) are
possible. We fix it with a type annotation on the return type:

```
def f (x: bool) : (#some bool | #none) = #just x
```

See Type Abbreviations for how to avoid typing long types in several places.

### 8.4.3. “The *x* operator may not be redefined”¶

The `&&`

and `||`

operators have magical short-circuiting
behaviour, and therefore may not be redefined. There is no way to
define your own short-circuiting operators.

### 8.4.4. “Unmatched cases in match expression”¶

Futhark requires `match`

expressions to be *exhaustive* - that is,
cover all possible forms of the value being matched. Example:

```
def f (x: i32) =
match x case 0 -> false
case 1 -> true
```

Usually this is an actual bug, and you fix it by adding the missing
cases. But sometimes you *know* that the missing cases will never
actually occur at run-time. To satisfy the type checker, you can turn
the final case into a wildcard that matches anything:

```
def f (x: i32) =
match x case 0 -> false
case _ -> true
```

Alternatively, you can add a wildcard case that explicitly asserts that it should never happen:

```
def f (x: i32) =
match x case 0 -> false
case 1 -> true
case _ -> assert false false
```

See here for details on how to use `assert`

.

### 8.4.5. “Full type of *x* is not known at this point”¶

When performing a record update, the type of the field we are updating must be known. This restriction is based on a limitation in the type type checker, so the notion of “known” is a bit subtle:

```
def f r : {x:i32} = r with x = 0
```

Even though the return type annotation disambiguates the type, this
program still fails to type check. This is because the return type is
not consulted until *after* the body of the function has been checked.
The solution is to put a type annotation on the parameter instead:

```
def f (r : {x:i32}) = r with x = 0
```