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Two Syntax Design Problems in Futhark and Their Resolution

Posted on December 9, 2016 by Troels Henriksen

Futhark will never be anyone’s primary language. Neither will Futhark be the primary language in any large system. As a pure functional language, any interaction with the outside world is impossible. As an array language, many problems are impractical to express directly in Futhark, unless some preprocessing is performed. Futhark’s intended niche is implementing performance-sensitive computational kernels, which are merely part of a larger system. Two design principles arise from this basic assumption:

  1. First, the compiler must generate code that is inter-operable with other languages. This is primarily a technical consideration in the compiler implementation.
  2. Second, the Futhark language must be designed to feel familiar. Futhark does not promise wide enough applicability to justify learning complicated and arcane notation and semantics.

The second principle is the topic of this post. I will discuss two recent design issues in the Futhark syntax, and how they were resolved. In one case, adherence to familiar notation was judged important enough to justify a rather odd lexical rule, while in another case, no existing notation was judged sufficient for our purposes. In both cases, Fritz Henglein provided invaluable input that forced rigour in our reasoning.

Futhark is primarily concerned with efficient compilation, and worrying about syntax may appear as unproductive procrastination. However, Futhark is not just a research study - it is a language built for practical use. As the main user interface to our compiler, the ergonomics of the language deserve attention. And more importantly, language design is fun.

Array Types

The first issue I wish to discuss is the design of Futhark’s syntax for array types. For a long while, we had simply adopted Haskell’s list type syntax: an array of i32s was written [i32], and a two-dimensional array of i32s was written [[i32]]. This was both familiar and readable. However, some timer later we extended Futhark with optional shape declarations, by which individual dimensions of an array could be annotated with a variable or constant indicating its size. For example, a function for matrix multiplication could be defined as:

let (x: [[i32,m],n]) (y: [[i32,p],m] y): [[i32,p],n] = ...

This nicely describes the invariant that an n*m matrix can be multiplied with an m*p matrix, yielding an n*p matrix. Here, m, n, and p are in binding position in the parameters. Shape declarations not only serve as documentation; they also provide useful hints to the compiler. In the future, we may even use them to signal compile-time errors when shapes are mismatched (using lightweight dependent types), but for now, they are checked at run-time.

Apart from having an effect on the aesthetics and readability of code, the notation for types is also important when communicating the semantics of the language. For example, using this notation for array types, we can describe the type of map as:

map : (a -> b) -> [a,n] -> [b,n]

This type concisely describes that the output array of map has the same outer size as the input array, while nothing is said about any inner sizes (a and b can be any type, including arrays with their own shape declarations).

In general, any array type [t] could receive a shape declaration as [t,n], meaning “array of n values of type t”. The motivation for placing the shape declaration to the right of the element type was due to the intuition that such annotations were an optional “extra”. However, this design turned out to have a major disadvantage: array types had to be read right-to-left. For example, a three-dimensional integer matrix of size k*n*m would be written [[[i32,m],n],k]. This proved very confusing in practice.

One simple solution would be to simply move the size annotation to the left of the element type. Our matrix multiplication function would then be:

let (x: [n,[m,i32]]) (y: [m,[p,i32]] y): [n,[p,i32]] = ...

Much better! But while we are mucking about with the syntax anyway, we might also consider more radical changes. The Haskell notation works well when you have only a small number of dimensions, but becomes unwieldy fast. How many dimensions does [[[[[i32]]]]] have? It is also annoying that the element type is hidden all the way in the middle. Deeply nested lists of lists are uncommon in Haskell, but not particularly so in Futhark.

We considered a C-style syntax, such as i32[k][n][m]. This looks less noisy, but unfortunately composes badly. When adding a new outer dimension (or stripping the outermost), we have to make modifications in the “middle” of the type. This makes it harder to give type signatures. Let us consider map again:

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

Here, ds and ds' are some arbitrary (possibly empty) list of shape declarations. This is very awkward! Things are much simpler if we move the dimensions to the left of the element type: [k][n][m]i32. Now, for any type t, an array of n ts is just [n]t The type of map can be written as:

map : (a -> b) -> [n]a -> [n]b

This has much cleaner properties of composition, and is the solution we ended up picking. But can we simplify further? As a notational shortcut we could permit [k,n,m]i32 instead of [k][n][m]i32. This saves a few characters, but we judged that it was not worth having two ways of expressing the same type. Especially since this notation is also much less readable when the optional dimension declarations are elided: compare [][][]i32 and [,,]i32.

Although the [k][n][m]i32 notation looks weird at first glance, it has been quite comfortable to use in practice. After a few months of trials, we have not run into any unfortunate issues. It is easy to type, too, as you don’t have to move the cursor around too much when adding or removing dimensions.

Array Indexing

The second issue appeared much more recently, and I am still unsure about the elegance of its solution.

In Futhark we support literal array expressions by enclosing the elements in brackets: [1,2,3]. We also support array indexing using the familiar notation of a name followed by indices in brackets: a[x]. So far, so good.

In its original incarnation, Futhark required parentheses around the arguments in a function call, like most languages derived from C. Recently, in order to ease equational reasoning and bring Futhark more in line with functional language conventions, the application syntax was changed to use juxtaposition. For example, f(x,y) became f x y. But a problem arose: how should f [x] be interpreted? Is it an application of the function f to the array [x], or an indexing of the array f at position x?

While applications of functions to literal arrays are rare in application code, they are common in tests and examples. When demonstrating map, it is convenient to be able to say:

map (+2) [1,2,3] == [3,4,5]

instead of:

map (+2) ([1,2,3]) == [3,4,5]

It was therefore not desirable to outright ban literal arrays as arguments to functions. Based on how indexing and application is used in practice, we can construct a disambiguation rule: an index expression may not have whitespace between the array name and the indices. Thus, f[x] is an index expression, and f [x] is a function application.

While this solves the ambiguity, anyone with parser writing experience probably feels a little uneasy at this solution. The reason is that such whitespace information is typically not available during syntax analysis, having been removed by the lexer. While there are good reasons for using hand-written parsers, the Futhark compiler uses a conventional lex+yacc setup, in part to ensure that the grammar remains simple and unambiguous.

The solution we chose is indisputably a lexer hack, but it has been surprisingly unproblematic. We simply introduced a new lexeme that represents a name followed immediately by a bracket. Thus, f[ is treated as a single undivided lexeme. To permit indexing of arbitrary parenthesises expressions, e.g. (f a b)[0], we also added )[ as a lexeme. The resulting grammar productions look a bit weird, but they are fully unambiguous, which gives us confidence in their robustness. However, it means that anywhere else these character sequences are valid (presently nowhere), we will have to handle these “conjoined” lexemes. This is a risk we take in order to support familiar-looking syntax.

This issue could have been solved in other ways. For example, we could have changed array literals to require a prefix, or maybe another form of brackets. We could also make radical changes to array indexing syntax. Maybe require a field access dot as in F#: a.[i]. Or perhaps treat arrays as functions and index via application: a i. These notations all have advantages and disadvantages, and we may revisit the issue in the future. For now, we have chosen to go with familiarity, at the cost of a parsing hack.