[Haskell-cafe] The 13-line example in Text.Megaparsec.Expr

[Jeffrey Brown]

I'm trying this again! The comment for Megaparsec.makeExprParser refers to
"parens". When I grep for "parens" in Megaparsec I get only two occurences,
both of them in comments:

    ./Lexer.hs:-- > parens    = between (symbol "(") (symbol ")")
    ./Expr.hs:-- > term = parens expr <|> integer <?> "term"

If I try defining parens as in the comment from Lexer.hs, I get these
errors:

    <interactive>:37:33:
        Couldn't match type ‘Char’ with ‘()’
        Expected type: [()]
          Actual type: [Char]
        In the first argument of ‘symbol’, namely ‘"("’
        In the first argument of ‘between’, namely ‘(symbol "(")’
        In the expression: between (symbol "(") (symbol ")")

    <interactive>:37:46:
        Couldn't match type ‘Char’ with ‘()’
        Expected type: [()]
          Actual type: [Char]
        In the first argument of ‘symbol’, namely ‘")"’
        In the second argument of ‘between’, namely ‘(symbol ")")’
        In the expression: between (symbol "(") (symbol ")")

[1]
https://hackage.haskell.org/package/megaparsec-5.0.1/docs/Text-Megaparsec-Expr.html

On Mon, Feb 29, 2016 at 2:05 AM, Özgür Akgün <ozgurakgun at gmail.com> wrote:

>
> On 29 February 2016 at 08:22, Stephen Tetley <stephen.tetley at gmail.com>
> wrote:
>
>>
>> For the expression parser, Megaparsec's documentation is wrong[*] and
>> probably it should use symbol rather than reservedOp. Note that symbol
>> is slightly different in Megaparsec as it's a plain combinator (rather
>> than one instantiated from a first class module as in Parsec) so it
>> takes two args rather than one.
>>
>>
>> [*] Well, likely wrong - I haven't got round to using Megaparsec yet.
>>
>
>
> You are right: https://github.com/mrkkrp/megaparsec/commit/
> 750adb7c70392c3195eda12d816f4a1a2305321e
>
>
> --
> Özgür Akgün
>
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>


-- 
Jeff Brown | Jeffrey Benjamin Brown
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