[commit: ghc] ghc-8.0: Add "ticks-exhausted" comment (e035680)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : ghc-8.0
Link       : http://ghc.haskell.org/trac/ghc/changeset/e03568010a4b2155d1afe67534e2d97fe560317b/ghc

>---------------------------------------------------------------

commit e03568010a4b2155d1afe67534e2d97fe560317b
Author: Simon Peyton Jones <simonpj at microsoft.com>
Date:   Tue Jan 26 09:24:33 2016 +0000

    Add "ticks-exhausted" comment
    
    This code deliberately builds a subtle negative-occurrence-of-data-type
    example, described in the paper, so with -O it'll give "simplifier
    ticks exhausted".
    
    This patch just adds a comment to explain.
    
    (cherry picked from commit 47b3f58889caa71bf096a149e58c2a9b94b75a7d)


>---------------------------------------------------------------

e03568010a4b2155d1afe67534e2d97fe560317b
 testsuite/tests/dependent/should_compile/dynamic-paper.hs | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/testsuite/tests/dependent/should_compile/dynamic-paper.hs b/testsuite/tests/dependent/should_compile/dynamic-paper.hs
index 4e89209..fd63871 100644
--- a/testsuite/tests/dependent/should_compile/dynamic-paper.hs
+++ b/testsuite/tests/dependent/should_compile/dynamic-paper.hs
@@ -1,6 +1,9 @@
 {- This is the code extracted from "A reflection on types", by Simon PJ,
 Stephanie Weirich, Richard Eisenberg, and Dimitrios Vytiniotis, 2016. -}
 
+-- NB: it includes a negative-recursive function (see delta1), and
+-- so will give "simplifer ticks exhausted", at least with -O
+
 {-#  LANGUAGE RankNTypes, PolyKinds, TypeOperators,
              ScopedTypeVariables, GADTs, FlexibleInstances,
              UndecidableInstances, RebindableSyntax,
@@ -239,6 +242,10 @@ tcMaybe  = TyCon  { tc_module  = Module  { mod_pkg   = "base"
 rt = undefined
 
 delta1 :: Dynamic -> Dynamic
+-- NB: this function behaves like a negative-recursive data type
+-- and hence leads compiler into an infinite inlining loop,
+-- and we get "simplifier ticks exhausted".
+-- See Section 7 of the paper "A reflection on types"
 delta1 dn = case fromDynamic dn of
              Just f   -> f dn
              Nothing  -> dn