[Haskell-cafe] Re: let vs. where

Dan Piponi wrote:

On Nov 13, 2007 1:24 PM, Ryan Ingram 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. -- Dan

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)))

[...snip...]

-- 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

-- Chris