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 lists.
One solution was already suggested: to make the comprehension syntax be pure syntactic sugar whose semantics depends on the semantics of the identifiers 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. -Jan-Willem Maessen
So you could keep the current list-only comprehension as default, and allow monad comprehension by providing a library (which the users need to import so as to hide the Prelude's definition).
Stefan
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