$ operator in Haskell gives data constructor error

Question

1:([2]) works as expected.

1:$[2] gives <interactive>:15:2: Not in scope: data constructor `:$'

I thought the $ operator parenthesizes everything after it: Haskell: difference between . (dot) and $ (dollar sign)

What is going on?

Answer 1

You put $ between a function and a value, and it applies the function to the value.

1: isn't a function, but (1:) is, so you can do (1:) $ [2] but not 1: $ [2].

(The error you got was because without spaces, the compiler thinks :$ is one thing, not two, and operators starting with : are data constructors, just like functions starting with capitals are data constructors.)

Answer 2

The $ operator is not syntax, it's just a normal function like every other. When you write

1 :$ [2]

The first problem the compiler sees is that :$ appears as its own operator (consider + + versus ++, these are very different things), but :$ is not defined anywhere.

If you were to write

1 : $ [2]

Then the compiler doesn't understand what to do since you have two operators right next to each other, this isn't allowed, just as 1 + * 2 isn't allowed. These expressions simply don't make any sense. The $ operator is actually just defined as

f $ x = f x

But it has a low precedence, like Please Excuse My Dear Aunt Sally for arithmetic operator precedence, so that you can chain operations more easily. It does not actually insert parentheses into an expression.