Proposal: Left-Associative Semigroup Operator Alias in "Data.Semigroup"

Daniel Bergey bergey at
Mon Jun 6 16:48:01 UTC 2016

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'm trying to understand the proposal to put this in Data.Semigroup
rather than in `pretty`

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/<>/></ which is easy, but requires dropping support for old
base / pretty, or CPP to define >< 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.



On 2016-06-06 at 07:30, Herbert Valerio Riedel <hvriedel at> 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
> for more details.
> Discussion period: 4 weeks
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