Hi Chris,
I've heard Simon (Peyton-Jones) twice now mention the desire to be able to embed a monadic subexpression into a monad.
I think this is a fantastic idea, please do so!
$( expr ) -- conflicts with template haskell ( <- expr ) -- makes sense, and I think it's unambiguous
Other ideas:
``expr`` -- back-ticks make sense for UNIX shell scripters (| expr |) -- I don't think anything uses this yet
This final (| one |) looks way too much like template haskell, it has the feel of template haskell, even if it isn't yet in the syntax. Your (<- proposal) feels a bit like an operator section - I'm not sure if that is a good thing or a bad thing, but for some reason feels slightly clunky and high-syntax overhead, perhaps because of the inevitable space between the <- and expr, and that ()<- are all fairly high semantic value currently in Haskell, while this extension should blend in, rather than stand out. The `` syntax is clever, and I like it, but I worry that its quite a long way from the current use of ` as infix, although I'm not sure if that is a particular issue given - (negation/subtraction) and -- (comment) couldn't be more different. Thanks Neil