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

[Henning Thielemann]

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