On Tue May 3 8:50 , Felipe Almeida Lessa sent:

On Tue, May 3, 2011 at 12:32 PM, Patrick LeBoutillier patrick.leboutillier@gmail.com> wrote:

...

I'm trying to understand this example and I can't get the types to line up. Can you provide the "real" types for ds and (applyOp op x y)?

This is what I'm working out:

    fmap ((:) ds) (applyOp op x y) == fmap (ds :) (applyOp op x y) == fmap (\dss -> ds : dss) (applyOp op x y)

     (applyOp op x y) >>= (flip (:) ds) == (applyOp op x y) >>= (: ds) == (applyOp op x y) >>= (\d -> d : ds)

To my untrained eyes is doesn't even look like the code is doing the same thing... What am I missing here (or more probably where is my mistake)?

I think that OP forgot a return, i.e., he meant 'applyOp op x y >>= return . flip (:) ds'.

You are correct, I did forget the return. Sorry for the confusion. This particular bit of code was pulled out of the middle of a RPN calculator. So 'ds' is a list of doubles. applyOp takes 2 numbers and emits a 'm double' which we then want to extract and add to the top of the list. The 'm' is there because I do my calculation in the Either monad to support unknown operators, invalid numbers, etc. A left value in the bind call stops the execution. To my still novice eyes 'foo >>= handleResult' is more readable than 'fmap handleResult foo'. When I look at the fmap call it looks like handlResult is the more important computation.