Re: [Haskell-beginners] typeclass error

21 Apr 2009

      Right, I was incorrect, you should use the solution that Quentin gave- 
to recap it,

The basic problem is that GHCi is trying to infer a single type -- 
called "monomorphic" -- for your more general type.
the type of `bar` is `Foo a => a -> a`, when ghci looks at a literal 
number (like 10) it interprets it as a "default" type of
Integer (not int). Since Integer does not instance Foo, then GHCi yells 
at you.

Quentin explained it far better than I, I refer you to his email.

/Joe

Amitava Shee wrote:
...
Thank you.
I get the following error after making the suggested changes
-- foo.hs
{-# LANGUAGE FlexibleInstances #-}
module Foo where
class Foo a where
    bar :: a -> a
--    
-- instance Foo Int where
--     bar i = i + 10
--    
-- instance Foo [Char] where
--     bar m = "foo" ++ m
--
instance Num a => Foo a where
    bar x = x + 10
-----
foo.hs:14:0:
    Constraint is no smaller than the instance head
      in the constraint: Num a
    (Use -XUndecidableInstances to permit this)
    In the instance declaration for `Foo a'
Failed, modules loaded: none.
Prelude>
...Amitava
On Tue, Apr 21, 2009 at 9:42 AM, Joe Fredette mailto:jfredett@gmail.com> wrote:
If I'm not mistaken, changing:
bar i = i + 10
to
bar i = i + 10::Int
ought to fix it.
The issue is that the compiler is being to general wrt the type of
    "10", it's inferring that it's just of type (Num a => a) -- the
    type of all numbers (sortof.)
Alternately, you could implement this as:
instance Num a => Foo a where
         bar x = x + 10
which similarly ought to fix it. Do you understand why?
Amitava Shee wrote:
I have the following code
-- foo.hs
        module Foo where
class Foo a where
           bar :: a -> a
          instance Foo Int where
           bar i = i + 10
          ------------------
It fails to compile withe following error
        *Foo> bar 10
<interactive>:1:4:
           Ambiguous type variable `t' in the constraints:
             `Num t' arising from the literal `10' at <interactive>:1:4-5
             `Foo t' arising from a use of `bar' at <interactive>:1:0-5
           Probable fix: add a type signature that fixes these type
        variable(s)
        *Foo>
- Thanks
        Amitava Shee
        ------------------------------------------------------------------------
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