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Futhark 0.15.1 released - now with size types!

Posted on March 15, 2020 by Troels Henriksen

Being cloistered up has led to a new release of Futhark being out (full changelog). This is a very large release, in particular with respect to the user-visible source language, mostly because of the addition of size types to Futhark. These have been the subject of a previous blog post, and I will get to them in a moment, but let me first mention two other major improvements.

Parallel safety checking

The parallel code generators can now handle bounds checking and similar safety checks in GPU code, including checking for integer division by zero! Only the exceptional impact of size types keeps this from being the highlight of the release, as having to constantly use unsafe has been an embarrassment for years.

The precise implementation technique is certainly going to be the subject of an upcoming blog post (and hopefully paper), but let me mention that the overhead of full safety checking seems to be about 10% on average. You can of course still use unsafe if 10% too much for you. Considering how relatively easy this was to implement, we should have done it years ago.

Better type errors

Type error messages are now a lot better. Instead of emitting a mismatch from deep within algorithm J, the compiler can in most cases provide an error that is similar to what you’d expect in a language without type inference. I still think Futhark has a way to go before it has good error messages, but they are a lot better than they used to be. Some examples (without source locations):

As with safety checking, we should have improved this years ago. It was surprisingly pleasant to work on as well, although I found it hard to come up with precise principles to follow when generating type errors. One thing I did do was stop considering unification as responsible for generating full type errors, and instead simply consider it a detector (and possibly elaborator when the precise error is obscure). This can be seen in the f example above: if the error occurs while checking a function application, then the error message primarily talks about how the function cannot be applied to the given argument, and shows the actual (pre-inference) types involved. If the actual type mismatch is more complex, for example deep inside nested records, then the actual unification error will also be shown.

Size types

Futhark now 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 =
  map (+) a b

This says that f takes two arrays of the same size. We use a size parameter, [n], to explicitly quantify the 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: i32) (x: t): [n]t = ...

An application replicate 10 0 will then have type [10]i32.

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 (covered below) to convert an unknown size to a known size.

Generally, unknown sizes are constructed whenever the true size cannot be expressed. The language reference lists all such cases, but here are some of the most interesting ones:

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.

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.

Complex slicing

Most complex array slicing, such as a[i:j], will have an unknown size. Exceptions are specially detected patterns such as a[0:j], which will have size j.

Complex ranges

Most complex ranges, such as a..<b, will have an unknown size. Again, a few patterns like 0..1..<n are detected specially for convenience.

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.

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.

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.

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, and the argument to a function is always evaluated before the function.

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.

In practice, this is implemented as a pass after size inference, where it is checked that all sizes are “created” before they are used as an implicit size parameter.

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 proram 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.

Sizes and 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 instiation 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-anomymous 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).

Are size types usable in practice?

When adding new type restrictions to a programming language, one has to be sure that the new, more restricted language is still useful for its intended purpose. Size types are a potential problem in this regard, because they are a novel and exotic feature in a language that isn’t supposed to be particularly difficult or advanced. However, evaluating the impact on tens of thousands of lines of Futhark code, including the benchmark suite and many Lys programs, shows that size types to a large extent just formalise what was already common and good programming practice. There are still cases where the error messages are obscure, but a lot of the work on improving type errors was motivated by making size types friendlier to work with. We have reasonable confidence that size types will be comprehensible to most Futhark programmers, including novices.