[Haskell-cafe] a monad for secret information

[Bulat Ziganshin]

Hello Felipe,

Wednesday, October 11, 2006, 4:47:59 AM, you wrote:

> Why does this works? "Yet Another Haskell Tutorial" teaches that
> pattern matching occurs at one stage and guard processing at other,
> and that there's no back (page 94).

something is definitely wrong - either book or your understanding. it's
very widely used feature, i just took a quick scan of Hugs Prelude and
found a lot of its uses:

(!!)             :: [a] -> Int -> a
xs     !! n | n<0 = error "Prelude.!!: negative index"
[]     !! _       = error "Prelude.!!: index too large"
(x:_)  !! 0       = x
(_:xs) !! n       = xs !! (n-1)

lexmatch (x:xs) (y:ys) | x == y  =  lexmatch xs ys
lexmatch xs     ys               =  (xs,ys)

showLitChar c | c > '\DEL' = showChar '\\' .
                             protectEsc isDigit (shows (fromEnum c))
showLitChar '\DEL'         = showString "\\DEL"

x ^ 0           = 1
x ^ n  | n > 0  = ....
_ ^ _           = error "Prelude.^: negative exponent"


i don't even say that i use this feature every day :)



-- 
Best regards,
 Bulat                            mailto:Bulat.Ziganshin at gmail.com