#5744: List layouts
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Reporter: nsch | Owner: nsch
Type: feature request | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.2.1
Keywords: | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: None/Unknown
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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Comment(by ehird):
Replying to [comment:11 nsch]:
> Replying to [comment:10 ehird]:
> > As far as desugaring goes, I would suggest that it reorder the
expressions (including flattening nested `mdo`s) to be right-associative,
and to avoid `mappend`ing things with `mempty`, e.g.
>
> Actually, this extension would never ever (!) use `mempty`, which is why
some have argued that `Monoid` may not exactly be the correct type class
for this. A `Semigroup` typeclass might be more appropriate:
>
> {{{
> class Semigroup a where
> gappend :: a -> a -> a
> }}}
>
> Which should satisfy associativity:
>
> {{{
> (a `gappend` b) `gappend` c = a `gappend` (b `gappend` c)
> }}}
>
> The `Semigroup` instance for monoids is trivial.
Fair enough; I'd stick with `Monoid` because it's already in base; ideally
[http://hackage.haskell.org/package/semigroups semigroups] would move in.
> This would translate to:
>
> {{{
> => mdo { as ... } `mappend` bs `mappend` ...
> => (as `mappend` ..) `mappend` bs `mappend` ...
> }}}
>
> Because of associativity this is equivalent to what you suggested.
My special case was intentional; while the two expressions must have
equivalent *results*, right-associative use of `mappend` is more efficient
for basically all `Monoid`s. This is why, e.g. RWH suggests using a
difference-list rather than `[]` as the monoid of a `Writer`.
> And by the way, `mdo` is already a reserved key word.
As I said:
> The only worry there is that it would clash with the obsolete recursive
do notation, but it *is* deprecated, and its history of being a reserved
keyword will mean that clashes are unlikely.
The new notation for recursive do-notation is `do rec`.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5744#comment:12>
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