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

[Bob Ippolito]

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 <joshuatfriedlander at gmail.com>
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 <fa-ml at ariis.it> 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))
>>
>> instead.
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