Hi,
There is some discussion about the different design choices relevant
for Haskell's class system in the following paper:
"Type classes: exploring the design space"
Simon Peyton Jones, Mark Jones, Erik Meijer
Presented at the 1997 Haskell Workshop.
Section 4.5 discusses options related to the restrictions on the instance heads.
-Iavor
On Tue, Oct 14, 2008 at 7:32 PM, Derek Elkins
On Tue, 2008-10-14 at 19:20 -0700, George Pollard wrote:
I'm a little confused. Why is this allowed:
data Blah = Blah
instance Eq Blah where x == y = True
But not this:
class Stringable a where toString :: a -> String
instance Stringable [Char] where toString = id
(Resulting in:)
Illegal instance declaration for `Stringable [Char]' (All instance types must be of the form (T a1 ... an) where a1 ... an are distinct type *variables* Use -XFlexibleInstances if you want to disable this.) In the instance declaration for `Stringable [Char]'
'Blah' isn't a type variable, is it? Is my brain just not working right today?
Blah = T
for [Char], T = [] and a1 = Char where it should be a variable.
Why this is an error is basically because the Report says so.
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