7. Compiler Error Index

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

7.1. Uniqueness errors

7.1.1. “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

7.1.2. “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:

let 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:

let 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:

let 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.

7.1.3. “Unique-typed return value of x is aliased to y, which is not consumable”

This can be caused by a function like this:

let 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:

let f (xs: *[]i32) : *[]i32 = xs

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

7.1.4. “A unique-typed component of the return value of x is aliased to some other component”

Caused by programs like the following:

let 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:

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

7.2. Size errors

7.2.1. “Size x unused in pattern.”

Caused by expressions like this:

let [n] (y: i32) = x

And functions like this:

let 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.

7.2.2. “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:

let 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.