-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Tillmann Rendel wrote:
Hi Sylvain,
sylvain wrote:
the following code snippet
data Test = A Int instance Show Test main = print (A 1)
leads to:
*Main> main *** Exception: stack overflow
Since "instance Show Test" does not specify how method "show" should be implemented for datatype Test, "show = undefined" is assumed. Now, print calls show to create a string represention of (A 1), which produces the error message.
Actually, "show = undefined" isn't assumed. The problem is that the default definitions for show and showsPrec are: showsPrec _ x s = show x ++ s show x = showsPrec 0 x "" This allows you to define either one of showsPrec or show, and have a reasonable default definition for the other one. However, if you leave off the definition of _both_ of them, what you get is an infinite recursion (hence the stack overflow). - -- Micah J. Cowan Programmer, musician, typesetting enthusiast, gamer. GNU Maintainer: wget, screen, teseq http://micah.cowan.name/ -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.7 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFI79Bu7M8hyUobTrERAnTLAJ4i3jfeS8QQQBSXqBNVW0Ph77GYKQCeKzXn BPo7VBKIn/7t4Z8FoU+FbYc= =7ovS -----END PGP SIGNATURE-----