CODEWITHSUNDEEP
haskell
singpolyma
singpolyma

Reputation: 11262

Higher-order functions and ST

I'm playing with http://hackage.haskell.org/packages/archive/vault/0.2.0.0/doc/html/Data-Vault-ST.html and want to write functions like the following:

onVault  f = runST (f <$> Vault.newKey)
onVault2 f = runST (f <$> Vault.newKey <*> Vault.newKey)

etc. If I replace these functions with ones that take no arguments and call a particular function instead of f, it works, but these higher-order functions will not type check.

What is going on and can I fix it?

Upvotes: 3

Views: 140

Answers (1)

hammar
hammar

Reputation: 139930

You need to give onVault and onVault2 rank 2 types.

{-# LANGUAGE Rank2Types #-} -- RankNTypes would also work

onVault :: (forall s. Key s a -> b) -> b
onVault2 :: (forall s. Key s a -> Key s b -> c) -> c

This is because runST :: (forall s. ST s a) -> a requires that the passed action is polymorphic in the state thread parameter s, which is a type-level trick used to guarantee purity. See the ST monad article on HaskellWiki for details.

Upvotes: 3

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