Re: [Haskell-cafe] Forcing type conversion - probably FAQ(?)

1 Dec 2004


      On Wed, 1 Dec 2004, Dusan Kolar wrote:
...
Hello,
I'm trying to make work the following code (I'm using GHCi
with flags -fglasgow-exts -fallow-undecidable-instances):
infixl 6 :+, :-
infixl 6 .+, .-
data TMyExpr = TMyExpr :+ TMyExpr
             | TMyExpr :- TMyExpr
             | Val Int
             | Id String
What about introducing a single type parameter class which can convert
both Int and TMyExpr to TMyExpr?

(untested code follows)

class MkExpr a where
  toExpr :: a -> TMyExpr

(.+) :: (MkExpr a, MkExpr b) => a -> b -> TMyExpr
(.+) x y = toExpr x :+ toExpr y

Re: [Haskell-cafe] Forcing type conversion - probably FAQ(?)

Henning Thielemann