Type signature of readIntP seems redundant?

[David Feuer]

I would guess history has a lot to do with it. The whole readP mechanism is
pretty old, and many of our modern conventions hadn't really taken hold yet.

On Mon, Jun 8, 2020, 2:54 PM Andreas Abel <andreas.abel at ifi.lmu.de> wrote:

>  > readIntP :: Num a => a -> (Char -> Bool) -> (Char -> Int) -> ReadP a
>
> Dunno, but one could guess that people find this function more
> convenient to apply, in the spirit of
>
>    readIntP x (('0' <=) .&&. (<= '9')) ((- ord '0') . ord)
>
> One could always add your function, under a different name.
>
> They easily convert into each other, using a function like
>
>    boolToMaybe :: Bool -> a -> Maybe a
>    boolToMaybe True  a = Just a
>    boolToMaybe False _ = Nothing
>
> On 2020-06-06 01:04, Dannyu NDos wrote:
> > Current type signature of readIntP is:
> >
> > readIntP :: Num a => a -> (Char -> Bool) -> (Char -> Int) -> ReadP a
> >
> > This seems kinda redundant. It inputs two functions, one checking
> > whether a character is valid digit, and the other assigning an integer
> > to the digit.
> >
> > Shouldn't we merge them together to one function? Like this:
> >
> > importControl.Monad
> > importData.Foldable
> > importData.Maybe
> > importText.ParserCombinators.ReadP
> >
> > readIntP:: Numa=> a-> (Char-> MaybeInt) -> ReadPa
> > readIntP base valDigit = do
> > maybeNs <- manyTill (fmap (fmap fromIntegral . valDigit) get) $ do
> > str <- fmap (fmap valDigit) look
> > casestr of
> > []-> return ()
> > (Nothing:_) -> return ()
> > _ -> pfail
> > letn = foldl' (\l r -> casel of
> > Nothing -> r
> > Just l1 -> fmap (base * l1 +) r
> > ) Nothing maybeNs
> > guard (isJust n)
> > return (fromJust n)
> >
> >
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