Sounds like a Maybe to me...
On Tue, Apr 5, 2011 at 1:41 PM, Mike Meyer
One of the exercises in Real World Haskell is to write a glob matcher without translating it to regular expressions. This turns out to be a lot more subtle than it looks (character classes like [!]-}] tend to break things).
In doing this, I wrote a matchCharClass function for handling character classes. The initial version had type:
matchCharClass :: Pattern -> Char -> (Bool, Pattern)
I.e. - it returned rest of pattern after the char class as well as a match succeed/fail indicator. Upon reflection, I realized that only one of the two was ever used, so rewrote it to be:
matchCharClass :: Pattern -> Char -> Either Bool Pattern
This made the calling code a little larger - taking a apart tuples is a bit easier than taking apart Either's - but simplified the values being generated. I then realized that the code only used one of the Bool values: if it was True, then Pattern got used instead. So I rewrote it a third time, giving:
data CharClass = Fail | Pattern String matchCharClass :: Pattern -> Char -> CharClass
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 seen at http://pastebin.com/eyre8795 (as always, critiques welcome).
I'd like to hear what more experienced haskell programmers have to say about those three ways of returning multiple values.
Thanks,
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