schneegloeckchen:
Hello,
recently I read about pattern guards which are a candidate for Haskell'.
As I understand them, their purpose is to provide an easier way to write functions like (taken from the wiki, it does not run):
clunky env var1 var1 = case lookup env var1 of Nothing -> fail Just val1 -> case lookup env var2 of Nothing -> fail Just val2 -> val1 + val2 where fail = var1 + var2
Wouldn't
clunky :: Num a => [(a, a)] -> a -> a -> a clunky env var1 var2 = case (lookup var1 env, lookup var2 env) of (Just v1, Just v2) -> v1 + v2 _ -> var1 + var2
be simple enough to make a syntax change superflous? It is so simple, that there is no need for `fail` in Haskell or Haskell' :)
There's been a previous discussion on this[1]. The joy of pattern guards reveals once you have more conditions. Consider this real world example: clean_ s| Just _ <- no_io `matchRegex` s = "No IO allowed\n" | Just _ <- terminated `matchRegex` s = "Terminated\n" | Just _ <- hput `matchRegex` s = "Terminated\n" | Just _ <- outofmem `matchRegex` s = "Terminated\n" | Just _ <- stack_o_f `matchRegex` s = "Stack overflow\n" | Just _ <- loop `matchRegex` s = "Loop\n" | Just _ <- undef `matchRegex` s = "Undefined\n" | Just _ <- type_sig `matchRegex` s = "Add a type signature\n" | Just (_,m,_,_) <- ambiguous `matchRegexAll` s = m | Just (_,_,b,_) <- inaninst `matchRegexAll` s = clean_ b | Just (_,_,b,_) <- irc `matchRegexAll` s = clean_ b | Just (_,m,_,_) <- nomatch `matchRegexAll` s = m | Just (_,m,_,_) <- notinscope `matchRegexAll` s = m | Just (_,m,_,_) <- hsplugins `matchRegexAll` s = m | Just (a,_,_,_) <- columnnum `matchRegexAll` s = a | Just (a,_,_,_) <- extraargs `matchRegexAll` s = a | Just (_,_,b,_) <- filename' `matchRegexAll` s = clean_ b | Just (a,_,b,_) <- filename `matchRegexAll` s = a ++ clean_ b | Just (a,_,b,_) <- filepath `matchRegexAll` s = a ++ clean_ b | Just (a,_,b,_) <- runplugs `matchRegexAll` s = a ++ clean_ b | otherwise = s -- Don 1. http://www.haskell.org/pipermail/haskell-prime/2006-September/001689.html