mike_k_houghton at yahoo.co.uk
Fri Apr 14 19:12:14 UTC 2017
Thanks but I tried something like that before I posted. I’ll try again maybe I mistyped.
> On 14 Apr 2017, at 19:17, David McBride <toad3k at gmail.com> wrote:
> Try breaking it up into pieces. There a literal "package" which is
> dropped. There is a first identifier, then there are the rest of the
> identifiers (a list), then those two things are combined somehow (with
> literal "package" *> (:) <$> identifier <*> restOfIdentifiers
> restOfIdentifiers :: Applicative f => f [String]
> restOfIdentifiers = many ((:) <$> char '.' <*> identifier
> I have not tested this code, but it should be close to what you are looking for.
> On Fri, Apr 14, 2017 at 2:02 PM, mike h <mike_k_houghton at yahoo.co.uk> wrote:
>> I have
>> data PackageDec = Pkg String deriving Show
>> and a parser for it
>> packageP :: Parser PackageDec
>> packageP = do
>> literal “package"
>> x <- identifier
>> xs <- many ((:) <$> char '.' <*> identifier)
>> return $ Pkg . concat $ (x:xs)
>> so I’m parsing for this sort of string
>> “package some.sort.of.name”
>> and I’m trying to rewrite the packageP parser in applicative style. As a not quite correct start I have
>> packageP' :: Parser PackageDec
>> packageP' = literal "package" >> Pkg . concat <$> many ((:) <$> char '.' <*> identifier)
>> but I can’t see how to get the ‘first’ identifier into this sequence - i.e. the bit that corresponds to x <- identifier in the
>> monadic version.
>> in ghci
>> λ-> :t many ((:) <$> char '.' <*> identifier)
>> many ((:) <$> char '.' <*> identifier) :: Parser [[Char]]
>> so I think that somehow I need to get the ‘first’ identifier into a list just after Pkg . concat so that the whole list gets flattened and everybody is happy!
>> Any help appreciated.
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