On 7 June 2016 at 23:24, Daniel Bergey wrote:

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 >

I think one argument is that if it's in `pretty` it's in a single pretty-printing package; if it's in `base` somewhere then it's available for use with all pretty-printing packages. Note: I maintain wl-pprint-text, but in there I use the Data.Monoid version of <>. I admittedly haven't done any benchmarks to determine whether this is an issue or not.

A discussion[1] linked from the wiki suggests that the left-associative version *may* have better asymptotics. Is this confirmed?

Is this change mostly motivated by in-GHC use of `pretty`?

Is there a reason expressions like `a <> 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 wrote:

...

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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