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haskellidris

Haskell version of Idris !-notation (bang notation)

I've had the luxury of learning a bit of Idris lately and one thing I've found extremely convenient is the !-notation, which let's me shorten monadic code inside a do block such as

a' <- a
b' <- b
c' <- c
someFunction a' b' c'

to the much nicer

someFunction !a !b !c

Now when I write code in Haskell, I'm looking for something similar but as far as I can tell it doesn't exist (and the bang character is obviously already used for strict pattern matching). Is there any way to avoid having a bunch of trivial left arrows inside a do block? Perhaps an extension that adds a rewriting rule, or something to that effect?

Solution

  • Since every monad is an Applicative (with GHC >= 7.10) we can write

    someFunction <$> a <*> b <*> c

    Note that if someFunction returns a monadic value of type m T, the above will return m (m T), which is likely not what we want (as @pigworker points out below). We can however join the two layers together:

    join $ someFunction <$> a <*> b <*> c