syntax of ST monad declaration

Question

I recently began looking at core libraries on Hackage, and there's a recurring idiom I don't understand. Here's an example from the ST module:

instance Monad (ST s) where
    {-# INLINE (>>=)  #-}
    (>>) = (*>)
    (ST m) >>= k
      = ST (\ s ->
        case (m s) of { (# new_s, r #) ->
        case (k r) of { ST k2 ->
        (k2 new_s) }})

In particular, I don't understand (# new_s, r #). I assume the second hash refers to an unboxed value, but the rest is a mystery to me (something to do with "new state", presumably).

score 7 · Accepted Answer · answered Apr 07 '17 at 22:55

7

(# x, y, z #) is an unboxed tuple with three elements. See "8.2.2. Unboxed Tuples" at https://downloads.haskell.org/~ghc/6.8.3/docs/html/users_guide/primitives.html.

The rest is basically just an implementation of State.

answered Apr 07 '17 at 22:55

amalloy

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In general, `#` in Haskell means it is doing some sort of low level and/or primitive stuff. You must enable the `MagicHash` language extension to even use `#` in names. – Lazersmoke Apr 08 '17 at 00:46

syntax of ST monad declaration

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