[BUG] alter does not consume leading whitespace

[david-davies]

Description

alter does not consume leading whitespace according to the new whitespace parser.

Reproduction Steps

This issue was originally found in gigaparsec, and I do not have scala installed, so this will be in haskell.
Assume the lexical description has whitespaceIsContextDependent = True, and the default whitespace parser does not consider newlines (\n) to be whitespace.
Then consider this expression parser:

spaceOrNewLine = Just (\c -> c == ' ' || c == '\n')

Expr = ExprNum Int | ...

expr = number 
  <|> ...
  <|> ("(" *> alter spaceOrNewLine expr <* ")")

Plain numbers can be preceded by only spaces, whereas a parenthesised expression allows for newlines.

Expected behavior

I would expect the following file to parse:

(
   2 )

and produce ExprInt 2.
However, we instead get the error:

Error: (line 1, column 2):
  unexpected newline
  expected parentheses
  >(
    ^
  >  2 )

This is confusing, as we have changed the definition of whitespace, but so far whitespace trailing "(" has only been consumed using the old definition.
I would expect any trailing whitespace according to the new definition be consumed also.

Additional context

Parsley Version: Gigaparsec 0.3.0.0
Scala Version: GHC 8.10-9.8

Suggested Fix

The defn of the alter combinator,

wsImpl.rollback(wsImpl.setDuring(whiteSpace(newSpace))(within))

should probably be changed to

wsImpl.rollback(wsImpl.setDuring(whiteSpace(newSpace))(whiteSpace *> within))

or whatever the scala syntax for *> is.