[Haskell-beginners] Using output of head in data constuctor

[Francesco Ariis]

Hello Josh

Il 28 giugno 2020 alle 14:36 Josh Friedlander ha scritto:
> I want to create a log parser like this:
> 
> module LogAnalysis where
> import Log
> 
> parseMessage :: String -> LogMessage
> parseMessage xs
>   | length(words(xs)) < 3 = Unknown xs
>   | notElem(head(words(xs)) ["I", "E", "W"]) = Unknown xs
>   | otherwise = LogMessage Info 3 head(words(xs))
> 
> But GHC gives me "• Couldn't match type ‘[a0] -> a0’ with ‘[Char]’
>       Expected type: String
>         Actual type: [a0] -> a0"

I suspect `LogMessage Info 3 head(words(xs))` is the problem. This is
the same as writing

    LogMessage Info 3 head (words xs)

keeping in mind how whitespace and parentheses work in Haskell. You
probably want

    LogMessage Info 3 (head (words xs))

instead.