How to correctly qualify types for working with the (transformed) ST and random monads

Question

Here is my code:

...

import System.Random ( RandomGen, next, split )
import qualified Data.Array.MArray as MAI
import Data.Array.ST.Safe( STUArray )
import Control.Monad.ST.Safe(ST)
import qualified Control.Monad.Random as CMR
import Control.Monad.Trans.Class( lift )

data GraphEdgeYaml = GraphEdgeYaml { 
    specie1:: NodeName,
    specie2 :: NodeName,
    sign :: Int,
    speed :: Int
}

type LinksSTA s = STUArray s Int GraphEdgeYaml

-- Change a simple link 
swapLink :: RandomGen g => 
    LinksSTA s
    -> g
    -> ST s g
swapLink graph generator = 
   let
      swap_op :: CMR.RandT g (ST s) ()
      swap_op = do 
          (low_limit, high_limit) <- lift $ MAI.getBounds graph
          idx_value <- CMR.getRandomR (low_limit, high_limit)
          return ()
   in do
      (_, new_generator) <- CMR.runRandT  swap_op generator
      return new_generator

and here is the error message that I get:

hs/SignMatrixBuild/Randomize.hs:43:26:
    Could not deduce (RandomGen g1)
      arising from a use of `CMR.getRandomR'
    from the context (RandomGen g)
      bound by the type signature for
                 swapLink :: RandomGen g => LinksSTA s -> g -> ST s g
      at hs/SignMatrixBuild/Randomize.hs:(38,1)-(47,28)
    Possible fix:
      add (RandomGen g1) to the context of
        the type signature for swap_op :: CMR.RandT g1 (ST s1) ()
        or the type signature for
             swapLink :: RandomGen g => LinksSTA s -> g -> ST s g
    In a stmt of a 'do' block:
      idx_value <- CMR.getRandomR (low_limit, high_limit)
    In the expression:
      do { (low_limit, high_limit) <- lift $ MAI.getBounds graph;
           idx_value <- CMR.getRandomR (low_limit, high_limit);
           return () }
    In an equation for `swap_op':
        swap_op
          = do { (low_limit, high_limit) <- lift $ MAI.getBounds graph;
                 idx_value <- CMR.getRandomR (low_limit, high_limit);
                 return () }

How do I fix this?

Answer 1

One way to fix it is to bring the type variables s and g into scope using the ScopedTypeVariables extension, the other is to simply omit the local type signature on swap_op .

If the local signature is omitted, the type can be inferred -- that leaves, however, the problem of the constraint

MAI.MArray (STUArray s) GraphEdgeYaml (ST s)

that is needed. There are two options,

• change the array type to STArray
• use STUArray s indeed

If you change the array type to STArray , no constraint is needed (since there is an instance MArray (STArray s) e (ST s) that covers all element types), and without the local type signature it compiles without problems.

If you want to keep the STUArray s, the function can only be used where an instance is in scope. It is best, usually, to provide such instances either where the class is defined (not an option here) or where the type is defined (that would be this module).

So then you should write an

instance MAI.MArray (STUArray s) GraphEdgeYaml (ST s)

in that module, with that instance, the constraint would be fulfilled and need not be placed on the function. Note, however, that writing such an instance is not trivial.

Alternatively, you could add the constraint to the signature and load off the burden of defining an (orphan) instance to the user of swapLink .

I don't know what a NodeName is, but whether GraphEdgeYaml is an unboxable type seems doubtful. I would therefore recommend switching to STArray s.