Adding an ignore function to Control.Monad
lemming at henning-thielemann.de
Fri Aug 14 13:29:52 EDT 2009
On Thu, 11 Jun 2009, John Meacham wrote:
> On Thu, Jun 11, 2009 at 11:21:54PM +0100, Sittampalam, Ganesh wrote:
>> On the flip side, could you give some examples where ignoring return
>> values is useful and natural?
> certainly, the very useful parser example mentioned.
> (char 'x' >> char 'y') - we want to parse x, then y
Are you really interested to only ignore the result of the first 'char'
parser and maintain the result of the second one?
> (char 'x' `mplus` char 'y') - we want to parse one of x or y, returning
> which one we got so we can change behavior based on it.
Indeed using the same 'char' parser in both cases is convenient. It seems
that parser monads are different from IO monad, in that it is more common
to ignore results. (Or parser monads are developed with the simplicity of
ignoring results in mind.)
char :: Char -> Parser Char
ignore :: Functor m => m a -> m Ignore (or m ())
ignore = fmap (const Ignore)
we would have to write
ignore (char 'x') >> char 'y'
char 'x' `mplus` char 'y'
char :: Char -> Parser Ignore (or Parser ())
echo :: Functor m => (a -> m Ignore) -> (a -> m a)
echo f x = fmap (const x) $ f x
we would write
char 'x' >> char 'y'
echo char 'x' `mplus` echo char 'y'
If the parser library anticipates frequent use of ignore it could define a
class IgnorableChar a where
wrapChar :: Char -> a
class IgnorableChar Ignore where
wrapChar _ = Ignore
class IgnorableChar Char where
wrapChar = id
char :: IgnorableChar char => Char -> Parser char
Cumbersome to define, but as simple to use as today's 'char' with today's
'>>'. Just as a compromise for parser libraries.
Ok, all these suggestions make things more complicated. Convenience vs.
safety - It's certainly a matter of taste which one is more important for
> When using the 'Writer' monad or similar it comes up a whole lot.
> sometimes you are interested in the result, sometimes the written value,
> sometimes both.
I think one should use Monoid class directly whereever possible. I have
seen frequently that people use Writer monad instead of Monoid only
because of the do notation.
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