Haskell Curl Help -

ok, i'm trying wrap head around io in haskell, , figured i'd write short little app dealing web pages it. snippet i'm getting tripped @ (with apologies bobince, though fair, i'm not trying parse html here, extract 1 or 2 values):

titlefromurl url =     (_, page) <- curlgetstring url [curltimeout 60]        matchregex (mkregexwithopts "<title>(.*?)</title>" false true) page 

the above should take url in string form, scan page points matchregex, , return either nothing or just [a], a matched (possibly multi-line) string. frustrating thing when try doing

prelude> (_, page) <- curlgetstring url [curltimeout 60] prelude> matchregex (mkregexwithopts "<title>(.*?)</title>" false true) page 

in interpreter, precisely want to. when try load same expression, , associated imports file, gives me type inference error stating couldn't match expected type 'io b' against inferred type 'maybe [string]'. tells me i'm missing small , fundamental, can't figure out what. i've tried explicitly casting page string, that's programming superstition (and didn't work in case).

any hints?

yeah, ghci accepts sort of value. can say:

ghci> 4 4 ghci> print 4 4 

but 2 values (4 , print 4) not equal. magic ghc doing if typed evaluates io something executes action (and prints result if something not ()). if doesn't, calls show on value , prints that. anyway, magic not accessible program.

when say:

do foo <- bar :: io int    baz 

baz expected of type io something, , it's type error otherwise. let execute i/o , return pure value. can check noting desugaring above yields:

bar >>= (\foo -> baz) 

and

-- (specializing io simplicity) (>>=) :: io -> (a -> io b) -> io b 

therefore

bar :: io foo :: baz :: io b 

the way fix turn return value io value using return function:

return :: -> io  -- (again specialized io) 

your code then:

titlefromurl url =     (_, page) <- curlgetstring url [curltimeout 60]        return $ matchregex (mkregexwithopts "<title>(.*?)</title>" false true) page 

for of discussion above, can substitute monad io (eg. maybe, [], ...) , still true.