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

[Dusan Kolar]

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

class MkExpr a b where
  (.+) :: a -> b -> TMyExpr
  (.-) :: a -> b -> TMyExpr

instance MkExpr Int Int where
  (.+) a b = (Val a) :+ (Val b)
  (.-) a b = (Val a) :- (Val b)

instance MkExpr Int TMyExpr where
  (.+) a b = (Val a) :+ b
  (.-) a b = (Val a) :- b

instance MkExpr TMyExpr Int where
  (.+) a b = a :+ (Val b)
  (.-) a b = a :- (Val b)

instance MkExpr TMyExpr TMyExpr where
  (.+) a b = a :+ b
  (.-) a b = a :- b


It compiles fine, no problem. Nevertheless, writing

(3::Int) .+ (4::Int)

seems to me quite clumsy, but 3 .+ 4 does not work. :-(

 It says the 3 is from class Num and no variant is present
 for it, I know, it is not present, but how to force it to
 convert it to Int "automatically"?  When I try to incorporate
 instance (Num a, Num b) => MkExpr a b where
  (.+) a b = (Val . round a) :+ (Val . round b)
  (.-) a b = (Val . round a) :- (Val . round b)
 the situation is even worse. :-(


I tried several modifications, but they don't work either. :-\


I was trying to find the discussion (I wonder whether it
went on the similar topic through the list recently), but
failed. :-(

If someone can just point me to any resource I could study
the possible solutions - maybe I put wrong data to google. ;-)
Or just to stop me, if I'm trying to do impossible... :-\

Thx,
  Dusan