F# 中的 Haskell `$` 可能吗？

Question

In Haskell, we can write

print $ abs $ 3 - 5

using $ .

In F#, we can write

printfn "%d" << abs <| 3 - 5

However, in many cases in F#, it would be also useful to have the same functionality of $ since the above are simply expressions with binary operators.

The trick of Haskell $ is that has somewhat the lowest precedence among its binary operators.

So, I investigated again

https://docs.microsoft.com/en-us/dotnet/fsharp/language-reference/symbol-and-operator-reference/#operator-precedence

and I observe, unfortunately, that there are no operators in lower precedences that can overload safely because all of them seem essential.

Do you have any ideas on this?

Do you think Haskell $ in F# is possible??

Answer 1

Like Haskell, F♯ also allows you to define new infix operators, not just overload existing ones. Unlike Haskell however, you can't select precedence (aka fixity) at will, nor select any non-letter Unicode symbol. Instead, you must choose from ! , $ , % , & , * , + , - , . , / , < , = , > , ? , @ , ^ , | . The fixity is then determined by analogy with the standard operators, in a way I don't entirely understand but what does seem to hold is that for any single-symbol infix, you can make a custom one with the same fixity by adding a . in front.

So, to get a lowest-fixity operator you'd have to call it .| . However, | is left-associative, so you couldn't write printfn "%d".| abs.| 3 - 5 printfn "%d".| abs.| 3 - 5 printfn "%d".| abs.| 3 - 5 . However I'd note that in Haskell, your example would also rather be written print. abs $ 3 - 5 print. abs $ 3 - 5 , and that can indeed be expressed in F♯:

let (.|) f x = f x
printfn "%d" << abs .| 3 - 5

To transliterate print $ abs $ 3 - 5 , you'd need a right -associative operator. The lowest-precedence custom right-associative operator I can manage to define is .^ , which indeed gets the example working:

let (.^) f x = f x
printfn "%d" .^ abs .^ 3 - 5

However, this operator doesn't have very low precedence, in particular it actually has higher precedence than the composition operators!

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Really, you shouldn't be doing any of this and instead just use printfn "%d" << abs <| 3 - 5 printfn "%d" << abs <| 3 - 5 as you originally suggested. It's standard, and it also corresponds to the preferred style in Haskell.