Hi!
Can somebody explain to me why ghc/hugs fails to compile the following
Haskell program? As long as MyInt is not an instance of Num the
compilations should succed... but it don't. :-( Why?
$ cat multi.hs
data MyInt = MyInt Int deriving Show
class Op_plus a b where
plus :: a - b - Int
Hi!
Koen Claessen wrote:
| instance (Num a) = Op_plus a MyInt where
| i `plus` (MyInt b) = i + b
Remember that b is of type Int, but you also say that i is
of any Num type. This clashes, since + requires both if its
arguments to hve the same types.
You are right! I did't notice that error.
Hi!
Keith Wansbrough wrote:
instance Op_plus MyInt MyInt where
instance (Num a) = Op_plus a MyInt where
instance (Num a) = Op_plus MyInt a where
[..]
Overlapping instance declarations:
multi.hs:9: Op_plus a MyInt
multi.hs:12: Op_plus MyInt a
Failed, modules loaded: none.
The GHC manual talks
