Update withError associativity to match Control.Alt

[thomashoneyman]

Description of the change

Fixes #91, which resulted from purescript/purescript-control#79.

---

Checklist:

• Added the change to the changelog's "Unreleased" section with a link to this PR and your username
• Linked any existing issues or proposals that this pull request should close
• Updated or added relevant documentation in the README and/or documentation directory
• Added a test for the contribution (if applicable)

[thomashoneyman]

@JordanMartinez do I need to do anything so that the master branch of control is picked up in CI?

[JordanMartinez]

@thomashoneyman Looks like you need to bust the cache.

[JordanMartinez]

Hm.... Thoughts on how to design around the CI error?

[JordanMartinez]

I tried out Nate's suggestion of bumping prec to 2 for <?>. Here's what I get as I change that value:

prec set to 3 (current)

[1/1 TypesDoNotUnify] test/Examples.purs:94:26

  94          ( string " " <?> "Failed to match space as a separator"
                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  
  Could not match type
  
    String
  
  with type
  
    t0 t1
  
  while checking that type String
    is at least as general as type t0 t1
  while checking that expression "Failed to match space as a separator"
    has type t0 t1
  in value declaration extractWords
  
  where t0 is an unknown type
        t1 is an unknown type

prec set to 2 (Nate's idea): `string (" " "msg")`, not `(string " ") "msg"`

[1/1 TypesDoNotUnify] test/Examples.purs:94:26

  94          ( string " " <?> "Failed to match space as a separator"
                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  
  Could not match type
  
    String
  
  with type
  
    t0 t1
  
  while checking that type String
    is at least as general as type t0 t1
  while checking that expression "Failed to match space as a separator"
    has type t0 t1
  in value declaration extractWords
  
  where t0 is an unknown type
        t1 is an unknown type

prec set to 4 or 5

[1/1 TypesDoNotUnify] test/Examples.purs:102:19

  102                <?> "Failed to match period as a separator"
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  
  Could not match type
  
    String
  
  with type
  
    Parser t0
  
  while checking that type String
    is at least as general as type Parser t0
  while checking that expression "Failed to match period as a separator"
    has type Parser t0
  in value declaration extractWords
  
  where t0 is an unknown type

prec set to 6

[1/1 TypesDoNotUnify] src/StringParser/CodeUnits.purs:80:63

  80  char c = satisfy (_ == c) <?> "Could not match character " <> show c
                                                                    ^^^^^^
  
  Could not match type
  
    String
  
  with type
  
    Parser Char
  
  while checking that type String
    is at least as general as type Parser Char
  while checking that expression show c
    has type Parser Char
  in value declaration char

[JordanMartinez]

Since <|> is right associative now, it may be better to resolve this situation by dropping <?> entirely:

foo <|> fail "failed due to bar"

[JordanMartinez]

Actually, the error is caused by what is likely just weird code string " " <?> "foo" <|> string "other". <?> is expected to go at the end of a chain. Here, it goes in front. Moreover, string " " <?> "foo" <|> string "bar" is the same as string " " <|> string "bar" since the error message will be ignored by the <|>:

extractWords :: Parser (NonEmptyList String)
extractWords = do
  endBy1 (regex "[a-zA-Z]+")
    -- try commenting out one of the "<|> string ..." lines and see what happens
    ( many1
        ( string " " <?> "Failed to match space as a separator"
                  -- ^^^
            <|> string "'"
              <?> "Failed to match single-quote char as a separator"
            <|> string ","
              <?> "Failed to match comma as a separator"
            <|> string "?"
              <?> "Failed to match question mark as a separator"
            <|> string "."
              <?> "Failed to match period as a separator"
              <?> "Could not find a character that separated the content..."
        )
    )

[JordanMartinez]

See purescript-contrib/purescript-parsing#164 (comment) and that thread's previous comments for context, but we need to keep <?> as infixl but drop its precedent to 4.