One more 'do' pattern
tov at ccs.neu.edu
Sat Mar 28 13:04:27 EDT 2009
Bulat Ziganshin wrote:
>> • The syntax gains very little over the nice consistent syntax we
>> already have – all you do is move a symbol a little to the left.
> action x y >>= \v -> do
> action x y $ \v -> do
One way to settle this kind of dispute would be a real macro system.
Bulat could define and use the desired syntax without modifying the
language definition in a way that would disturb others. Template
Haskell is great for some things, but it's unsuitable and unsatisfying
in a case like this.
This is out of scope for Haskell', of course, but it's something the
community should consider adding at some point.
I've uploaded a package called preprocessor-tools to Hackage that
provides very quick-and-dirty syntax extension using a preprocessor. I
used it to define a do-notation for parameterized monads, back before
GHC supported that.
At one point I used it to define syntax for a "continuation let", which
binds the "result" of a CPS-style function :
clet P = E1 in E === E1 (\P -> E)
I think this is what Bulat wants. (Bulat, if you want to try this, let
me know and I'll try to resurrect the code.)
 It seems to generalize nicely:
clet P1 = E1
Pn = En
E1 (\P1 -> ... -> En (\Pn -> E) ... )
flip runCont id $ do
P1 <- Cont E1
Pn <- Cont En
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