[Haskell-cafe] Those damned parentheses

[Antoine Latter]

On Sat, May 7, 2011 at 2:10 PM, Eitan Goldshtrom <thesourceofx at gmail.com> wrote:
> Hi. I am kind of tired of all of the parentheses I have to put in places and
> I'm trying to figure out what is the correct way to write code such that I
> can leave out parentheses. For example, I have the following:
>
> data Message = ... --leaving this out because it's not important
> data Plane = Plane {
>   id          :: Int,
>   position    :: (Int,Int,Int),
>   direction   :: Int,
>   path        :: [Int],
>   messagebuf  :: Chan Message
>   }
>
> main = do
>   c <- newChan :: Chan Message
>   p <- Plane 0 (0,0,0) 0 [] c
>   f p
>
> f p = putStrLn $ (show Main.id p) ++ " - message received"
>

One thing to keep in mind is that function application binds tightest,
and function application goes from left to right, so:

> a b c d

is parsed as:

> ((a b) c) d

I'm not really sure what you're doing, but I would probably write it as:

> f p = putStrLn $ show (Main.id p) ++ " - message received"

Because function application binds tightest, I don't need parenthesis
around the (show ...) part.

The other rule to note is the the ($) function binds the weakest of
them all, so you can do a lot to the left of it and to the right of it
without parentheses.

> This causes an error "The function `show' is applied to two arguments". If I
> put instead:
> f p = putStrLn $ (show . Main.id p) ++ " - message received"
>
> I get the error "Couldn't match expected type `[Char]' with actual type `a0
> -> c0'". The only way it seems to work is
> f p = putStrLn $ (show (Main.id p)) ++ " - message received"
>
> This seems to be the same for many other situations where I try to use
> function composition of some sort. It's just getting kind of annoying.
>
> -Eitan
>
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe
>
>