Fuse zipWith3

I was surprised to discover that zipWith3 has no RULES associated with it to aid fusion.

Can we add these?

I have no idea what I'm doing, but would this substitution work:

{-# RULES "zipWith3"
zipWith3 f a b c = zipWith id (zipWith f a b) c
#-}

Those who know what they're doing will likely devise a better strategy.

Trac metadata

Trac field	Value
Version	8.4.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

mentioned in issue #14037 (closed)

changed milestone to %8.6.1

changed weight to 5

added Tbug Trac import labels

If you are going to do that, you might as well define zipWith3 in that way to begin with?

In fact, defining zipWith3 in this way and adding an INLINE pragma causes it to fuse nicely with map.

My example program and the resulting core here - https://gist.github.com/mpickering/c190fc25260a3460f4633d0248659192

I noticed that zipWith itself has an explicit recursive definition, even though the rule right below it universally applies and zaps it to a build expression. I do not know the implications of these different implementation choices.

changed milestone to %8.8.1

These won't be addressed in GHC 8.6.

assigned to @trac-TDecki

Trac metadata

Trac field	Value
Differential revisions	- → D5241

Related Ticket also wants to do this.

Trac metadata

Trac field	Value
Related	- → #14037 (closed)

mentioned in commit fb669f51

closed

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

added Pnormal label