Possible to implement something like Haskell's syntax for writing arbitrary / custom infix functions in user space?

[ym-han]

Haskell allows users to define custom infix operators (see, e.g., https://jplahn.gitbooks.io/a-happy-haskell/content/functions/infix_operators.html for examples).

Is it possible to make a Langium grammar, or extend the Langium parser, to similarly allow users to declare / define custom infix notation? (or to do perhaps a nearby but weaker variant of this?)

[msujew]

Hey @ym-han,

I think that should be possible purely by implementing additional parser rules. Kind of similar to the arithmetics example, you would have a new kind of expression:

langium/examples/arithmetics/src/language-server/arithmetics.langium

Lines 25 to 26 in b2a6fe9

	Addition infers Expression:
	Multiplication ({infer BinaryExpression.left=current} operator=('+' | '-') right=Multiplication)*;

InfixFunctionCallExpression infers Expression: 
  Multiplication 
  ({infer InfixFunctionCallExpression.left=current} 
    // The "operator" is actually a reference to a function
    operator=[Function:BacktickString] 
    right=Multiplication
  )*;

terminal BacktickString: /`[^`]*`/;

Note that you can replace BacktickString with a different terminal, depending on your language. For example, AFAIK haskell also allows something like this:

<+> :: Num a => a -> a -> a
<+> b = a + b

ghci> 1 <+> 1
2

That would require something like this:

operator=[Function:InfixOperatorText] 
...

InfixOperatorText returns string: AnyOperator | BacktickString;

terminal AnyOperator: /(\!#\$%&\*\+\.\/<=>\?@\\^\|-~)+/;
terminal BacktickString: /`[^`]*`/;

[ym-han]

Very nice, thank you!!!!

I'm guessing things like fixity declarations are not going to be possible, but most DSLs probably don't require that