[Haskell-cafe] Re: let vs. where
haskell at list.mightyreason.com
Tue Nov 13 19:05:01 EST 2007
Dan Piponi wrote:
> On Nov 13, 2007 1:24 PM, Ryan Ingram <ryani.spam at gmail.com> wrote:
>> I tend to prefer where, but I think that guards & function declarations are
>> more readable than giant if-thens and case constructs.
> Up until yesterday I had presumed that guards only applied to
> functions. But I was poking about in the Random module and discovered
> that you can write things like
> a | x > 1 = 1
> | x < -1 = -1
> | otherwise = x
> where 'a' clearly isn't a function. Seems like a nice readable format
> to use. Probably everyone except me already knew this already though.
I recalled having used this trick in the regex-tdfa regular expression matching
engine. There is an option for single-line vs multi-line matching that changes
whether ^ and $ get tested against '\n'. By using this trick I was able to
decide which matching to use once and that decision gets cached:
> matchHere regexIn offsetIn prevIn inputIn = ans where
> ans = if subCapture then runHerePure else noCap
> where subCapture = captureGroups (regex_execOptions regexIn)
> && (1<=rangeSize (bounds (regex_groups regexIn)))
> -- Select which style of ^ $ tests are performed.
> test | multiline (regex_compOptions regexIn) = test_multiline
> | otherwise = test_singleline
> where test_multiline Test_BOL _off prev _input = prev == '\n'
> test_multiline Test_EOL _off _prev input = case input of
>  -> True
> (next:_) -> next == '\n'
> test_singleline Test_BOL off _prev _input = off == 0
> test_singleline Test_EOL _off _prev input = null input
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