Make it possible to evaluate monadic actions when assigning
record fields
Neil Mitchell
ndmitchell at gmail.com
Thu Jul 12 07:10:10 EDT 2007
Hi
> Put differently, I don't see a compelling use-case for the proposed
> syntax extension. But I've seen many misused monads.
A compelling use-case:
http://darcs.haskell.org/yhc/src/libraries/core/Yhc/Core/Simplify.hs
Look at coreSimplifyExprUniqueExt
And from that file:
-- helpers, ' is yes, _ is no
coreCase__ x y = f $ CoreCase x y ; coreCase_' x y = f .
CoreCase x =<< y
coreLet__ x y = f $ CoreLet x y ; coreLet_' x y = f .
CoreLet x =<< y
coreLam__ x y = f $ CoreLam x y ; coreLam_' x y = f .
CoreLam x =<< y
coreApp__ x y = f $ CoreApp x y ; coreApp'_ x y = f . flip
CoreApp y =<< x
i.e. i've manually defined ' and _ variants to thread monadic effects
through in quite horrible ways. The monad in question simply supplies
free variables, so could be applied in any order.
I think with this extension I can define:
coreCase x y = f $ CoreCase x y
coreLet x y = f $ CoreLet x y
...
And taking just one rule, before:
f (CoreApp (CoreLet bind xs) ys) = coreLet_' bind (coreApp__ xs ys)
After:
f (CoreApp (CoreLet bind xs) ys) = coreLet bind $(coreApp xs ys)
Much nicer!
This extension seems like a great idea - my only concern would be
about the order of computations. Clearly left-to-right makes sense,
but this may break some natural intuition in Haskell:
flip f a b == f b a
flip f $(a) $(b) /= f $(b) $(a)
I don't think that is a show stopper though.
Thanks
Neil
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