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

Parentheses in Haskell aren’t really related to function application, they are only for grouping. It makes more sense if you avoid using them unless strictly necessary. In Haskell instead of `f(g(x))` we would write `f (g x)`, and instead of `f(x,g(y),z)` we would write `f x (g y) z`. You could use more parentheses but it would be more confusing, such as `(f)(x)(g(y))(z)`. On Sun, Jun 28, 2020 at 05:50 Josh Friedlander wrote:

Thanks Francesco, that works. I don't quite understand what the issue was, though. Specifically: - Did the parentheses around (xs) hurt, or were they just redundant? - Wouldn't the parentheses around (head ...) be binding it as an argument to whatever comes before (in this case, 3)?

On Sun, 28 Jun 2020 at 14:47, Francesco Ariis wrote:

...

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))

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