On Thu, 07 Oct 2010 18:03:48 +0100, Peter Wortmann
On Tue, 2010-10-05 at 17:10 -0700, Evan Laforge wrote:
+1 for something to solve the "dummy <- m; case dummy of" problem. Here are the possibilities I can think of:
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
Reasons: * "<-" is already "sugary", and the transformation is similar. Just removes the need for the user to define a throw-away name. * Better than liftMX and the Applicative operators. As shown, this is more flexible while requiring less magic operators as a bonus. Also makes more clear where the sides effects actually are. * Goes well with the spirit of getting the good parts of imperative coding where it potentially makes the code more concise. Can be abused, obviously, but I have also seen a lot of code that I feel could be written better using this.
Anything I am overlooking here? I tried to find a discussion about something like this, but didn't really know what to look for...
Your notation feels very tempting, however it relies a lot on finding the "do" to put the bind. Recall that "do" is just syntax, and that it has no more meaning than its desugaring. 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} Imagine that "b" can be equal to "b1 >> b2" and so where placing the "x <- c" is non obvious and it should be. On the other hand case (<- m) of {...} being translated into m >>= \x -> case x of {...} is non-ambigous. Best regards, -- Nicolas Pouillard http://nicolaspouillard.fr