GHC rewrite rules pragma

[Simon Peyton-Jones]

| This is the main wibble people forget when writing rules -- inlining.
| In your example, 'gen' is so cheap, it is immediately
| inlined, so it won't be available to match on in your rule.

I'll add a note in the user manual about this.

In general, GHC tries RULES before inlining.  In this particular example that fails for a tiresome internal reason -- but in any case the solution Don mentions is noticeably more robust.

Simon

|
| Anything you want to match should have its inlining delayed:
|
|     {-# OPTIONS -O2 -fglasgow-exts #-}
|
|     gen :: [a] -> a
|     gen = head
|     {-# INLINE [1] gen #-}
|
|     {-# RULES
|        "gen/Char" gen = spec
|       #-}
|
|     spec :: [Char] -> Char
|     spec x = 'x'
|
|     main :: IO ()
|     main = putStr (gen "aaa":"\n")
|
| Running this in ghc-core, we see:
|
|     RuleFired
|         1 gen/Char
|
| Good.
| And the program prints out:
|
|     $ ./A
|     x
|
| There was some discussion of adding a warning for any function
| used in the left hand side of a rewrite rule, that doesn't have
| precise inlining control added. That would have caught this.
|
| -- Don
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