As a newbie of Haskell, I find the life becomes easier once I understand function application:
(1) function application is actually “function call”. For example, define a simple add function who returns the sum of 2 numbers:
# cat add.hs
add :: Num a => a -> a -> a
add a b = a + b
Load it in ghci, and call this function:
# ghci
GHCi, version 8.2.2: http://www.haskell.org/ghc/ :? for help
Prelude> :l add
[1 of 1] Compiling Main ( add.hs, interpreted )
Ok, one module loaded.
*Main> add 2 4
6
*Main> add 3 6
9
Beware that the tokens in function application are separated by space. So once you see following format:
a b ..
You know it is a function application, and also a “function call”.
(2) function application has the highest precedence. Check following example:
*Main> add 1 2 ^ add 1 2
27
It is equal to “(add 1 2) ^ (add 1 2)” literally.
(3) $ operator is “application operator”, and it is right associative, and has lowest precedence. Check following instance:
*Main> add 1 $ add 2 $ add 3 4
10
The $ operator divides the expression into 3 parts: “add 1“, “add 2” and add 3 4. Because $ is right associative, the result of add 3 4 is fed into add 2function first; then the result of add 2 $ add 3 4 is passed into add 1. It is equal to “add 1 ( add 2 ( add 3 4 ) )” in fact, so $ can be used to remove parentheses.
References:
Prelude;
Calling functions.
