Haskell "where" indentation: why must it be indented past identifier?

Question

This code:

import Data.Char (digitToInt)  myInt :: String -> Int myInt [] = error "bad input: empty string" myInt (x:xs)   | x == '-'  = -1 * myInt xs   | otherwise = foldl convert 0 (x:xs)   where convert acc x         | x `elem` ['0'..'9'] = 10 * acc + digitToInt x         | otherwise           = error ("bad input: not an int - " ++ [x]) 

Fails:

Prelude> :l safeListFs.hs
[1 of 1] Compiling Main ( safeListFs.hs, interpreted )

safeListFs.hs:9:8: parse error (possibly incorrect indentation)
Failed, modules loaded: none.

But this version:

import Data.Char (digitToInt)  myInt :: String -> Int myInt [] = error "bad input: empty string" myInt (x:xs)   | x == '-'  = -1 * myInt xs   | otherwise = foldl convert 0 (x:xs)   where convert acc x           | x `elem` ['0'..'9'] = 10 * acc + digitToInt x           | otherwise           = error ("bad input: not an int - " ++ [x]) 

is ok:

Prelude> :l safeListFs.hs
[1 of 1] Compiling Main ( safeListFs.hs, interpreted )
Ok, modules loaded: Main.

I can't figure out why those two last indents matter.

Answer 1

Basically, Haskell notes the column where the first non-space character after where appears (in this case, the c of convert) and treats following lines beginning in that column as new definitions inside the where.

A line that continues the definition of the previous line (such as your | guards) must be indented to the right of the first non-space character (c in your code).

A line indented to the left of c would be outside the where (for example, the start of your next top-level function).

It's the column of the first character following where that is crucial, even if it's on a new line:

  where     convert acc x       | ...     anotherFunction x y      ^