(moving to haskell-cafe) Alexandre Weffort Thenorio writes: | Ooops a small error before but here is the right one. | | Great. I got almost everything. My problem now is: | | I got a function called findstr where | | findstr "aaaa" "zzzzz" ["xxxxaaaa","xxxaaaaxxx"] = | ["xxxxzzzzz","xxxzzzzzxxx"] with the inferred type findstr :: String -> String -> [String] -> [String] | and then I try something like | | fillIn lines = do | bkfilled <- (findstr str str2 lines) with the inferred type (findstr str str2 lines) :: [String] Note that [] (the list data constructor) is a monad. So, this 'do' expression is in the [] monad, where perhaps you intended the IO monad. Computations in the [] monad have the effect of iterating over the elements of lists. So, the bound variable gets the inferred type bkfilled :: String which you've noticed in the error message. | write bkfilled | | where write takes a [String] and concatenates it writing it to a file then. | But I get this error saying: | | Expected Type: [String] | Inferred Type: String | In the first argument of 'write' namelly 'bkfilled' | In a do expection pattern binding: write bkfilled | | Any idea?? I mean bkfilled is supposed to be [String] but it says it is a | String, any idea why??? The smallest code change to fix this is to add a 'return', which will wrap another monad (in this case, IO) around findstr's [String] result...

fillIn lines = do bkfilled <- return (findstr str str2 lines) write bkfilled

... but you should be able to simplify that code, using one of the monad laws in http://haskell.org/onlinereport/basic.html#sect6.3.6 Regards, Tom