Why the backslash is needed?

[txo]

I’m quite a noob so sorry for this silly question. I can’t get why backslash before function in this statement is needed.
map (\n -> n + 1) [1, 2, 3, 4, 5]

Why is this not sufficient?
map (n -> n + 1) [1, 2, 3, 4, 5]

Thank you.

[chexxor]

One reason might be for parsing efficiency.

In (n -> n + 1), it’s not clear it’s a function until ->, at which point the parser needs to back-track to the open parentheses and re-parse it as arguments to a lambda expression and a lambda body, rather than as a “normal” expression.

In (\n -> n +), it’s clear from the \ that the following should be parsed as arguments to a lambda, so no back-tracking is required, which can be expensive.

[chexxor]

Also, I think it’s necessary for delimiting where the arguments to the lambda start, as \a b c -> a + b + c is a legal lambda expression.

[natefaubion]

The parser has to lookahead with \ anyway because \ is also a valid operator. I think the answer is simply because PS is a Haskell derivative and Haskell uses \ as an ASCII lambda symbol.