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

[Lauri Alanko]

On Thu, Oct 07, 2010 at 02:45:58PM -0700, Nicolas Pouillard wrote:
> On Thu, 07 Oct 2010 18:03:48 +0100, Peter Wortmann <scpmw at leeds.ac.uk> wrote:
> > Might be off-topic here, but I have wondered for a while why Haskell
> > doesn't support something like follows:
> > 
> >   do case (<- m) of ...
> > 
> > With the more general rule being:
> > 
> >   do ... e (<- m) g
> >     =>
> >   ... m >>= \tmp -> e tmp g

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. Frankly, that seems like a very
attractive way to make the do-notation into a more practical
imperative sub-language.

This should probably also cover binding, i.e. do { p <- C[(<- m)];
... } should also work.

> Imagine these examples:
> 
> do {a; b (<- c) d; e} => do {a; x <- c; b x d; e}
> 
> do {a >> b (<- c) d; e}
>   |
>   +--> do {x <- c; a >> b x d; e}
>   |
>   +--> do {a; x <- c; b x d; e}

To my understanding no rule would produce this latter variant. do {a;
b} is transformed into a >> do {b}, not the other way around. The
proposed transformation rule seems clear to me: the context covers the
entire expression-statement, including of course expressions
containing monadic operations:

do {a >> b (<- c) d; e}  =>  c >>= \x -> a >> b x d >> e

and if you want a to go before c, you have to do

do {a; b (<- c) d; e)    =>  a >> c >>= \x -> b x d >> e

> Imagine that "b" can be equal to "b1 >> b2" and so where placing the
> "x <- c" is non obvious and it should be.

I don't see what this has to do with anything. All we are interested
in is the syntax of do-expressions. The do-transformation is
completely oblivious to monadic operations within the statements, it
only produces some more monadic operations.

Cheers,


Lauri