Parser does not allow literal patterns

[Munksgaard]

According to the language reference, Futhark allows literals as patterns, meaning that the following code should be accepted:

def foo 1 = 1

However, the current parser does not allow this:

Error at program.fut:1:9-9:
Unexpected token: '1'
Expected one of the following: '='

If you find this error message confusing, uninformative, or wrong, please open an issue:
https://github.com/diku-dk/futhark/issues

Playground link

[athas]

Two solutions are possible: either change the grammar in the language reference, or allow this in the parser and instead fail during type checking.

The reason for this is that we don't want to allow refutable patterns in let bindings and function parameters. However, that also means we currently don't allow things like def f (#foo x) = ... where #foo is from a single-constructor type. That hardly comes up frequently, but we might want to allow it.

[Munksgaard]

Wouldn't allowing literals in function definition patterns introduce ambiguity here:

def a - 1 = 2

That could be interpreted both as an infix binding of the - operator or as a function a that matches only on the literal pattern -1.

[athas]

We just pick one of them, and since this would be rejected in the type checker anyway, the distinction is not important.