[Haskell-cafe] Re: Re: Lambda-case / lambda-if

Peter Wortmann scpmw at leeds.ac.uk
Fri Oct 8 10:09:21 EDT 2010

On Fri, 2010-10-08 at 01:13 +0300, Lauri Alanko wrote: 
> Your "general" rule doesn't subsume your case example, since a case
> expression is not an application. I think you mean something like
>  do C[(<- m)]
> =>
>  m >>= \tmp -> C[tmp]
> where C is an arbitrary expression context. It could further be
> generalized to allow several (<- ...) subterms in an expression, with
> implied left-to right sequencing.

Yes, that's what I meant. Thanks for describing it properly.

On Fri, 2010-10-08 at 05:41 -0700, Nicolas Pouillard wrote:
> Imagine find this code:
> do s1
>    C[(<- s2)]
> And I don't see the (<- s2) in C, and so I refactor it as:
> s1 >> C[(<- s2)]

This is roughly what I meant with "abused": Where "C" is very complex,
it might become non-obvious where exactly the monad actions are supposed
to happen. Hence such traps when refactoring.

Also of note: Just moving sub-expressions around isn't something that is
guaranteed to be save. Introducing new names and using them in "s2"
would be problematic, for example:

  do map (\x -> (<- putStrLn x)) ["a", "b"]

Obviously can't be made to work. You might have to check for this - or
maybe even disallow the shorthand inside lamdbas and lets. Might be less
satisfying to have such special cases, but it is still a good bit more
general than what is available right now.

  Peter Wortmann

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