On Wed, 06 Apr 2011 11:24:45 +0200
Christian Maeder
Am 05.04.2011 19:41, schrieb Mike Meyer: [...]
data CharClass = Fail | Pattern String matchCharClass :: Pattern -> Char -> CharClass
As mentioned by Alex Rozenshteyn this CharClass is isomorphic to "Maybe String".
I think I missed the mail from Alex. But yes, that makes sense. And learning how to use the built-in types is part of learning the language.
This only required minor changes to the code, but made it easy to add "Error String" to the CharClass datatype later. That version can be
Replacing Fail (or Nothing) by "Error String" is like going to "Either String String".
Ok this confuses me. What does "or Nothing" by "Error String" mean? data CharClass = Fail | Pattern String | Error String is the same as "Maybe (Either Pattern Error)"? (Fail being a poorly chosen name - NoMatch would be better).
Yet, user-defined data types (no type synonyms!) may increase readability (and type safety). However, one disadvantage is that some type class instances have to be redefined (or derived) if needed.
Right. I prefer adding the Pattern & Error types, so they document which of Left and Right should be used here.
"Maybe" and "Either String" are fairly standard (and have Monad and what not instances), still your data type CharClass is perfect (if it serves the purpose). HTH Christian
Yup. Very educational. Thanks.
seen at http://pastebin.com/eyre8795 (as always, critiques welcome).
This has been updated to use the 'Maybe (Either Pattern Error)" return
values.
Thank you.