CODEWITHSUNDEEP
haskelltypes
Clinton
Clinton

Reputation: 23145

How to write type signature for Data.Vector

The following code compiles on GHC-7.8.3, but I can't figure out how to write a explicit type signature for f:

import qualified Data.Vector.Generic.Mutable
import Data.Vector.Generic (freeze, thaw, Mutable, Vector)
import Control.Monad.Primitive (PrimState, PrimMonad)
import Control.Monad.ST (runST, ST)

f v = Data.Vector.Generic.Mutable.read v 0 >>= 
      Data.Vector.Generic.Mutable.write v 0 >> 
      return v

g :: Vector v a => v a -> v a
g v = runST $ (thaw v) >>= f >>= freeze

main = return ()

f here is just a function that does some work on a mutable vector and returns a mutable vector. I've done a dummy read and write just to force the type to be a vector.

So how can I write a type signature for f?

Note: I've significantly edited and the question to clarify and simplify it.

Upvotes: 3

Views: 140

Answers (1)

user2407038
user2407038

Reputation: 14623

That type for f doesn't really make sense, precisely because type families are non injective (which is exactly what it tells you). You are saying f takes a Mutable v (PrimState m) a, but that cannot determine what v actually is - this is the meaning of non-injective. Then, the output must depend on v. But again you don't know what v is.

In this case the solution is obvious: give f the type Monad m => a -> m a. But in general, you have to pass the type v to the function somehow. Typically you would write 

f :: (PrimMonad m, Vector v a) 
  => proxy v -> Mutable v (PrimState m) a -> m (Mutable v (PrimState m) a)
f _ =  return

The inferred type of 

f v = read v 0 >>= 
      write v 0 >> 
      return v

is 

(MVector v a, PrimMonad m) => v (PrimState m) a -> m (v (PrimState m) a)

The argument to the function isn't a type family application. This is precisely the explicit type you should give it. The type v is known. If you were to try to give it the type

(PrimMonad m, Vector v a) => Mutable v (PrimState m) a -> m (Mutable v (PrimState m) a)

like before, you get the exact same error.

Upvotes: 2

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