[Haskell-cafe] Re: Why is $ right associative instead
jmaessen at alum.mit.edu
Wed Feb 8 08:15:19 EST 2006
On Feb 8, 2006, at 1:34 AM, Stefan Monnier wrote:
>> The trouble with monad comprehensions was that it became far too
>> easy to
>> write ambiguous programs, even when you thought you were just
>> working with
> One solution was already suggested: to make the comprehension
> syntax be pure
> syntactic sugar whose semantics depends on the semantics of the
> the syntactic sugar expands into.
OK. Which identifiers? I happen to want a version which always uses
"concatMap" (or, equivalently, monadic bind), and never, ever the
direct "efficient" translation. To get the efficient translation for
lists a la Wadler, though, this requires either a wrapper, so that
the comprehension runs at the type ([a] -> [a]) and gets applied to
 at the very end, or it requires heavy lifting from the compiler
(foldr/build and its kin as seen in GHC, phc, etc.).
When it was all tied to lists, it was easy to gloss over the details
of the machinery.
> So you could keep the current list-only comprehension as default,
> and allow
> monad comprehension by providing a library (which the users need to
> so as to hide the Prelude's definition).
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