[Haskell-beginners] Could not get parser ready

[David McBride]

The problem is that in p you are using do notation for bind, which uses the
Monad instance for bind (>>=) for ((->) String), because Parser is a type
alias for (String -> [(a, String)].  But then you are using your own return
which is not of the same type as Monad's return would be.  The way you have
it now your return considers the loose `a` as the Monad parameter, but do
notation considers it [(a, String)] instead.

You can either a) write your own bind that conforms to your type.

bind :: Parser a -> (a -> Parser b) -> Parser b
bind = undefined

p' :: Parser (Char, Char)
p' = item `bind` \x -> item `bind` \_ -> item `bind` \y -> Foo.return (x, y)

Or you can use the Monad return instead.  But the type is not what you
expect.

p :: String -> ([(Char, String)], [(Char, String)])
p = do x <- item
       item
       y <- item
       Prelude.return (x,y)





On Mon, Nov 6, 2017 at 3:15 AM, Marcus Manning <iconsize at gmail.com> wrote:

> type Parser a = String → [(a,String)]
>
> item :: Parser Char
> item = λinp → case inp of
>                             [] → []
>                             (x:xs) → [(x,xs)]
> failure :: Parser a
> failure = λinp → []
>
> return :: a → Parser a
> return v = λinp → [(v,inp)]
>
> (+++) :: Parser a → Parser a → Parser a
> p +++ q = λinp → case p inp of
>                                  [] → q inp
>                                  [(v,out)] → [(v,out)]
>
> parse :: Parser a → String → [(a,String)]
> parse p inp = p inp
>
> p :: Parser (Char,Char)
> p = do x ← item
>            item
>            y ← item
>            return (x,y)
>
> It is described in pages 189-216 in [1].
>
> [1] https://userpages.uni-koblenz.de/~laemmel/paradigms1011/resources/pdf/
> haskell.pdf
>
> I assume the bind operator (==>) was overwritten by
>
> (>>=) :: Parser a → (a → Parser b) → Parser b p
> >>= f = λinp → case parse p inp of
>                              [ ] → [ ]
>                              [ (v, out) ] → parse (f v) out
>
> in order to manipulate the do expr to make the p function work, right?
>
> 2017-11-05 21:56 GMT+01:00 Tobias Brandt <to_br at uni-bremen.de>:
>
>> Hey,
>>
>> can you show us your Parser definition?
>>
>> Cheers,
>> Tobias
>>
>> ----- Nachricht von Marcus Manning <iconsize at gmail.com> ---------
>>      Datum: Sun, 5 Nov 2017 18:51:57 +0100
>>        Von: Marcus Manning <iconsize at gmail.com>
>> Antwort an: The Haskell-Beginners Mailing List - Discussion of primarily
>> beginner-level topics related to Haskell <beginners at haskell.org>
>>    Betreff: [Haskell-beginners] Could not get parser ready
>>         An: beginners at haskell.org
>>
>> Hello,
>>
>> I follow the instructions of script [1] in order to set up a parser
>> functionality. But I' get into problems at page 202 with the code:
>>
>> p :: Parser (Char,Char)
>> p = do
>>              x ← item
>>              item
>>              y ← item
>>              return (x,y)
>>
>>
>> ghci and ghc throw errors:
>> Prelude> let p:: Parser (Char,Char); p = do {x <- item; item; y <- item;
>> return (x,y)}
>>
>> <interactive>:10:65: error:
>>     • Couldn't match type ‘[(Char, String)]’ with ‘Char’
>>       Expected type: String -> [((Char, Char), String)]
>>         Actual type: Parser ([(Char, String)], [(Char, String)])
>>     • In a stmt of a 'do' block: return (x, y)
>>       In the expression:
>>         do x <- item
>>            item
>>            y <- item
>>            return (x, y)
>>       In an equation for ‘p’:
>>           p = do x <- item
>>                  item
>>                  y <- item
>>                  ....
>> Did the semantics of do expr changed?
>>
>> [1] https://userpages.uni-koblenz.de/~laemmel/paradigms1011/reso
>> urces/pdf/haskell.pdf
>>
>> Cheers,
>>
>> iconfly.
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.orghttp://mail.haskell.org/cgi-bin/mailman
>> /listinfo/beginners
>>
>>
>>
>>
>> ----- Ende der Nachricht von Marcus Manning <iconsize at gmail.com> -----
>>
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/beginners/attachments/20171106/3bc7eacf/attachment.html>