Re: [Haskell-cafe] Confused by instances

2008/4/28 Fraser Wilson :

On Mon, Apr 28, 2008 at 10:50 PM, Brandon S. Allbery KF8NH wrote:

...

The format is instance [context =>] classname instance. Your classname is ValueClass. Your instance is a. a is not of the form (T a1 ... an).

But neither is instance (Show a) => Show [a] ...

Yes it is, it's just a weird looking T, (namely []). This works just as well: instance (Show a) => Show ([] a) ... (Unless that's not H98, but I think it is) Instances have to have concrete constructors at their heads is for technical reasons, namely you can't in general do type inference with unrestricted instances. It is a pattern-matching algorithm as you'd expect, but it goes *backwards*, from right to left; i.e it sees the pattern [a] and generates a new constraint Show a. This is contrary to the intuition that it has a big set of instances, and when it sees, say, Show Int it adds Show [Int]. The direction of the arrow can be misleading :-) To answer your other question, no, there is no list show hack. What is being complained about is that you're using a type synonym as an instance. If you just expand the synonym everything works fine. This is not specific to lists, nor show. You're just not allowed to use synonyms in instances (and as the compiler suggests, this restriction can be lifted with {-# LANGUAGE FlexibleInstances #-}). Luke