[Haskell-cafe] [Alternative] some/many narrative

[Gregory Crosswhite]

On Dec 15, 2011, at 4:29 PM, Chris Wong wrote:

>> Okay, so how about the following as a user narrative for some and many?
>> 
>> ...
> 
> I was in the middle of writing my own version of Applicative when I
> stumbled on this intense debate. Here's what I wrote for the
> documentation:
> 
> class (Applicative f, Monoid f) => Alternative f where
>    -- | Keep repeating the action (consuming its values) until it
> fails, and then return the values consumed.
>    --
>    -- [Warning]: This is only defined for actions that eventually fail
>    -- after being performed repeatedly, such as parsing. For pure values such
>    -- as 'Maybe', this will cause an infinite loop.
>    some :: f a -> f [a]
>    some v = ...
> 
>    -- | Similar to 'many', but if no values are consumed it returns
> 'empty' instead of @f []@.
>    --
>    -- [Warning]: This is only defined for actions that eventually fail
>    -- after being performed repeatedly, such as parsing. For pure values such
>    -- as 'Maybe', this will cause an infinite loop.
>    many :: f a -> f [a]
>    many v = ...
> 
> Warnings are repeated for emphasis :)


I think that merely putting up documentation along these lines would be a huge improvement, though it should also contain the formal definitions from the old documentation in it somewhere as well.

Cheers,
Greg
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