I sent this from the wrong address, and am resending. Appologies to
those on both mailing lists, who will get two copies.
I've often needed to use Semigroup <> and pretty-print <> in the same
module. Thanks for trying to improve the situation.
Are there other Semigroups where people want the right-associative > b <+> c` occur more often than
`a <+> b <> c`? Or is that just an artifact of the pretty-printing <>
being left-associative?
If I understand the proposed migration path, upgrading `pretty` without
changing my code would silently change semantics. Upgrading correctly
requires s/<>/>< locally. This leaves me with a funny
(><) operator that seems mostly historical, and still the surprising
behavior of `a <+> b >< c`.
This doesn't seem like an attractive migration for anywhere I use
pretty-printing. Maybe it's still the best answer for GHC internal use;
I can't tell.
cheers,
bergey
Footnotes:
[1] https://github.com/haskell/pretty/issues/30#issuecomment-161146748
On 2016-06-06 at 07:30, Herbert Valerio Riedel
Hello!
In short, the right-associative fixity of (Data.{Monoid,Semigroup}.<>) subtly conflicts with definitions of (<>) in pretty printing APIs, for which the left-associative variant is sometimes desirable. This subtle overloading of (<>) is error-prone, as one has to remember which version of (<>) is currently in scope in order to be able to reason about non-trivial expressions involving this operator.
This proposal is an attempt to resolve this unfortunate and confusing situation by completing the `Semigroup`/`Monoid` vocabulary with a standard left-associative alias. Please see
https://ghc.haskell.org/trac/ghc/wiki/Proposal/LeftAssocSemigroupOp
for more details.
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