How to fix "error: parse error on input ‘=’" in Haskell

Question

I'm just trying to see if WinGHCi works for Haskell programming, and I don't know the reason why this shows the error above.

I do not know if this is the right application to work on Haskell, if you know another one that can actually work, I would appreciate.

f (x:xs) = f ys ++ [x] ++ f zs
 where
 ys = [a | a ← xs, a ≤ x]
 zs = [b | b ← xs, b > x]

<interactive>:21:20: error:
    parse error on input ‘=’
    Perhaps you need a 'let' in a 'do' block?
    e.g. 'let x = 5' instead of 'x = 5'

You want to read the [GHCi manual](https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/ghci.html). Also possibly upgrade your Haskell installation if it's older than version 8. — n. m. could be an AI, Sep 05 '19 at 16:40
@n.m. not multi-line. Also, related question: https://stackoverflow.com/questions/18162420/cannot-enter-multiline-statements-in-ghci — AJF, Sep 05 '19 at 16:48
@AJFarmar `:set` +m perhaps? Note the question is from 2013. — n. m. could be an AI, Sep 05 '19 at 16:56
Does `:set +m` let you use `where`? My (admittedly few) attempts all ended with parse errors on `where`. — chepner, Sep 05 '19 at 17:13
@chepner it works but you need to put `where` on the first line: `foo = bar where bar = 42` — n. m. could be an AI, Sep 06 '19 at 03:31
Ah, because you need something that a line can't otherwise end with, right? — chepner, Sep 06 '19 at 11:31

score 2 · Answer 1 · answered Sep 05 '19 at 17:11

There are two intertwined issues: whether you need let to introduce a definition, and whether you can write a multiline definition.

First, let. Prior to 8.0, GHCi required a definition be introduced with let:

let f (x:xs) = ...

Starting in 8.0, you can drop the let, and GHCi will figure out what you are trying to do.

As for the multiline statement, there are two ways you can do this. One is to explicitly delimit your block with :} and :{:

Prelude> :{
Prelude| f (x:xs) = f ys ++ [x] ++ f zs
Prelude|  where
Prelude|  ys = [a | a <- xs, a <= x]
Prelude|  zs = [b | b <- xs, b > x]
Prelude| :}

The other, enabling multiline mode with :set +m, appears to be picker about what it accepts.

How to fix "error: parse error on input ‘=’" in Haskell

1 Answers1