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
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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