TextEncoding lacks Eq and Show instances

[bos]

When trying to debug a Handle encoding problem today, I noticed that although I can use hGetEncoding to get the encoding, I cannot find out what that encoding actually is, as there is neither an Eq nor a Show instance for TextEncoding.

[simonmar]

Fixed:

Mon Sep 13 16:44:59 BST 2010  Simon Marlow <marlowsd@gmail.com>
  * Add a suitable Show instance for TextEncoding (#4273)

I didn't add an Eq instance, but you can always get an approximate equality by comparing the results of show.

[bos]

Thanks. Any particular reason you didn't go for Eq too?

[simonmar]

Replying to bos:

Thanks. Any particular reason you didn't go for Eq too?

I wasn't completely sure what it should do. e.g. do you want an Iconv-backed UTF-8 codec to compare equal to the built-in UTF-8 codec? They are functionally identical, so perhaps they should. If so, then comparing the results of show would be sufficient. If not, then Eq is more tricky.

[bos]

I'd imagine that if they do the same thing, then they should compare as equal.

[judahj]

I'd prefer that we avoided an Eq instance. Eventually TextEncoding will be an exposed datatype which could be defined by other libraries than base. When that happens, there won't be any guarantee that two TextEncodings with the same name will actually do the same thing. So allowing comparisons with == will be misleading, I think.