I just accidentally discovered a function declaration syntax that I have never seen before
\> add :: Int -> Int -> Int = (+)
\> add 1 2
3
\> -- or with points
\> add' :: Int -> Int -> Int = \x y -> x + y
\> add' 3 4
7
\> -- multi line
\> :{
> add''
> :: Int -> Int -> Int
> = \x y
> -> x + y
> :}
\> add'' 5 6
11
How come i have never seen this anywhere?
It is using the grammar for pattern type signatures.
add1 (x :: Int) (y :: Int) = x + y
add2 (x :: Int) (y :: Int) :: Int = x + y
add3 = (+) :: Int -> Int -> Int
(add4 :: Int -> Int -> Int) = (+)
I’m a little surprised you don’t need the parens around your bare add (I do round my add4).
This is a variety of PatternSignatures I would say – except I’m struggling to find the reference for it. … Ah, here.
Pattern signatures are newer than most Haskell textbooks.
Moreover it is not like you can write (f :: Int -> Int -> Int) x y = x + y.
They go all the way back to 6.8.1 which was released in 2007. I think quite a few books were written after that.
Furthermore, by the time you write polymorphic functions: This is rejected:
g :: a -> a = \x -> x -- rejected
Generally, pattern bindings may not have type variables. (I don’t know why, but it’s documented.) Pattern bindings mean pattern = expr, and recall that a special case is var = expr such as g = \x -> x.
What you can do though is
g1 (x::a) = x::a
g2 = \(x::a) -> x::a
OK, fine, correction:
Pattern signatures are not part of GHC default until GHC2021. GHC2021 is newer than most textbooks.
Correction to your correction: Pattern signatures used to be part of GHC, and Hugs vintage 2006 – as a somewhat loosely documented extension. Then they got taken out of GHC; then put back in GHC2021. I think there’s work in hand to put back result signatures per my add2 above(?) But in a more limited form than was always supported in Hugs.
(I freely admit to having not kept up.)
Thank you for pointing out this polymorphic deficiency!