[Haskell-beginners] Tuples .vs Either Vs datatypes

Alex Rozenshteyn rpglover64 at gmail.com
Tue Apr 5 20:01:30 CEST 2011

Sounds like a Maybe to me...

On Tue, Apr 5, 2011 at 1:41 PM, Mike Meyer <mwm at mired.org> wrote:

> 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,
>      <mike
> --
> Mike Meyer <mwm at mired.org>
> http://www.mired.org/consulting.html
> Independent Software developer/SCM consultant, email for more information.
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          Alex R
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