Question

Quick question, what is wrong with this?

(get) :: [a] -> Int -> a   -- <- line 21
(x:xs) get 0 = x
(x:xs) get (n+1) = xs get n

ghci gives this error when I try to load the file that contains that code.

Prelude> :load ch6.hs 
[1 of 1] Compiling Main             ( ch6.hs, interpreted )

ch6.hs:21:0: Invalid type signature
Failed, modules loaded: none.

I’m trying to make get an infix operator.

Answer 1

You shouldn’t have parentheses around get, to start with. The syntax for the whole definition looks a bit off, though. I’m guessing you wanted something like this:

get :: [a] -> Int -> a
get (x:xs) 0 = x
get (x:xs) (n+1) = xs `get` n

Note the backticks around get in order to use it infix, which is necessary here because the rules for an alphanumeric identifier are different from operators: Operators are made of symbols, are are infix by default, and to write them without arguments or use them prefix, you put them in parentheses. Alphanumeric identifiers are prefix by default, and surrounding them with backticks lets you use them infix.

You can use the backticks on the left-hand side as well, if you want, but that looks a bit odd to my eye:

(x:xs) `get` 0 = x
(x:xs) `get` (n+1) = xs `get` n

Incidentally, the pattern syntax n+1 is deprecated, so you probably shouldn’t use that. Instead, do this:

(x:xs) `get` n = xs `get` (n - 1)