Hi,
I'm trying to work out how to handle a choice at runtime which
determines what instance of a State monad should be used. The choice
will dictate the internal state of the monad so different
implementations are needed for each. I've concocted a very simple
example to illustrate this (below) - but it doesn't compile because
ghc complains that my type is ambiguous arising from my use of
'fromSeq'. I can kind-of see what the compiler is complaining about,
I'm guessing because it is the internals of my type which dictate
which state Monad to use and it can't know that?
Thinking about it I tried making SeqType an instance of Sequence
class, but had no luck here.
I understand that haskell is static at compile time, so I'm looking
for something like a template solution in C++ (rather than a virtual
function style implementation). I see there are libraries out their
which do this, but I was wondering in my simple example if there was a
way of doing this without writing a load of boilerplate code in main
(this would get ugly very quickly if you had loads of choices). If
this is impossible does anyone have an example / advice of
implementing simple template style code in Haskell?
Any help or suggestions would be really appreciated.
Many Thanks,
Phil.
Thus just implements a state Monad which counts up from 1 to 10, using
either an Int or a Double depending on user choice. It's pointless of
course, but illustrates my point.
{-# LANGUAGE TypeSynonymInstances #-}
import Control.Monad.State.Strict
data SeqType = SeqDouble Double | SeqInt Int
class SequenceClass a where
nextSeq :: State a Int
fromSeq :: SeqType -> a
instance SequenceClass Int where
nextSeq = State $ \s -> (s,s+1)
fromSeq (SeqInt i) = i
fromSeq _ = 0
instance SequenceClass Double where
nextSeq = State $ \s -> (truncate s,s+1.0)
fromSeq (SeqDouble d) = d
fromSeq _ = 0.0
chooser :: String -> SeqType
chooser inStr | inStr == "Double" = SeqDouble 1.0
| inStr == "Int" = SeqInt 1
| otherwise = SeqInt 1
main :: IO()
main = do userInput <- getLine
let result = evalState (do replicateM 10 nextSeq) $ fromSeq
$ chooser userInput
print result
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