On Mon, Feb 14, 2005 at 12:36:22PM +0100, Remi Turk wrote:
Now that this topic is brought up...
Occasionally I would need to define recursive datatypes using an explicit fixed-point operator, such as:
data Fix f = In (f (Fix f)) deriving (Show, Eq) data L a x = Nil | Cons a x deriving (Show, Eq)
However, Haskell was not able to derive from Fix f any instances.
[snip]
This is rather unsatisfactory, because I would not be able to inspect values of type Fix f in the interpreter. Is there a way to get around this?
Funny this comes up at this time, as Fix was on-topic yesterday at #haskell. One way to make Fix an instance of Show/Eq is this (based on http://www.haskell.org/hawiki/PreludeExts, where Fix is called Rec):
It's even more funny - just yesterday I wanted to derive Show and Read for a similar datatype :) Best regards Tomasz -- Szukamy programisty C++ i Haskell'a: http://tinyurl.com/5mw4e