Static data and RULES

Simon Peyton Jones simonpj at
Thu Feb 16 23:35:32 UTC 2017

I don’t understand any of this.

However, RULES are allowed to match on data constructors and it would be nice to let that keep happening.

Why won’t it keep happening?  What is the problem you are trying to solve?  Why does the fast-path make it harder?

Maybe open a ticket?


From: ghc-devs [mailto:ghc-devs-bounces at] On Behalf Of David Feuer
Sent: 16 February 2017 22:13
To: Ben Gamari <bgamari at>; Reid Barton <rwbarton at>
Cc: ghc-devs <ghc-devs at>
Subject: Static data and RULES

Ben Gamari and Reid Barton are interested in making it cheaper for static data to pass through simplification. The basic idea is that if a term is already made entirely of data constructors and literals, then there's nothing left to optimize.

However, RULES are allowed to match on data constructors and it would be nice to let that keep happening. But on the other hand, RULES are apparently (according to Duncan Coutts) already broken for strict data constructors, because they have workers and wrappers.

My thought: let's allow phased INLINE and NOINLINE pragmas for data constructors. The default would be INLINE. The ~ phase choice would not be available: once inline, always inline.


For all constructors:

If a constructor is allowed by pragmas to inline in a certain phase, then in that phase terms built from it can be considered static. Once static, always static.

If a constructor is not allowed to inline in a certain phase, terms built from it will be considered non-static.

After demand analysis and worker/wrapper, all constructors are considered inline.

For strict constructors:

A strict constructor wrapper prohibited from inlining in a certain phase simply will not.

Strict constructor wrappers will all be allowed to inline after demand analysis and worker/wrapper. This matches the way we now handle wrappers actually created in that phase.


For GADT syntax, this is easy:

data Foo ... where
  {-# INLINE [1] Bar #-}
  Bar :: ...

For traditional syntax, I think it's probably best to pull the pragmas to the top:

{-# NOINLINE Quux #-}
data Baz ... = Quux ... | ...
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