GHC-7.8 warning on rules that may not fire

Simon Peyton Jones simonpj at
Fri Mar 14 17:05:35 UTC 2014

You may think they are fragile, but not as fragile as saying nothing and hoping for the best, which is *super*-fragile. You can't rely on rules to take priority, because the rule only fires if it matches, and it may only match if some other inlining has taken place.  (We tried that originally.)

I don't have any plans to change this, but of course am always open to well-worked out design proposals.


| -----Original Message-----
| From: Glasgow-haskell-users [mailto:glasgow-haskell-users-
| bounces at] On Behalf Of Henning Thielemann
| Sent: 14 March 2014 16:57
| To: GHC Users List
| Subject: GHC-7.8 warning on rules that may not fire
| With GHC-7.8 I get lots of warnings like
| src/Foo/Bar.hs:215:6: Warning:
|      Rule "foo" may never fire
|        because ‘bar’ might inline first
|      Probable fix: add an INLINE[n] or NOINLINE[n] pragma on ‘bar’
| So far I thought that rewrite RULES always have precedence to INLINE.
| Has this changed? I hesitate to follow the advice of adding phase
| numbers in bracket because I consider these phase numbers a quite
| fragile and non-modular solution (like precedence numbers for infix
| operators).
| I have found:
| Is this still considered for future developments?
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