[Haskell-cafe] type class constraints headache

[Ryan Ingram]

Perhaps this thought exercise will make things clear:

> class Show a => Foo a where
>    toFoo :: String -> a

> foos :: (Foo a) => [(String, a)]
> foos = map (\f -> (show f, f)) [toFoo "a", toFoo "b", toFoo "c"]

> data Foo1 = Foo1
> instance Show Foo1 where show _ = "1"
> instance Foo Foo1 where toFoo _ = Foo1
> data Foo2 = Foo2
> instance Show Foo2 where show _ = "2"
> instance Foo Foo2 where toFoo _ = Foo2

> exercise :: [String]
> exercise = map fst foos

Exercise for the reader: what should the contents of "exercise" be?

Keep in mind that your question is exactly the same as this one, from
the compiler's point of view.

  -- ryan

On Wed, Mar 3, 2010 at 10:48 PM, Marcus Uneson <marcus.uneson at gmail.com> wrote:
> Thanks. I realize there are many ways to make it compile.
> However, I am trying to understand the mechanism behind --
> why does the first example compile and what constraints does
> enumerateMethodNames add on a (which it does not inspect)?