Changes between Version 4 and Version 5 of MonomorphicPatternBindings

Timestamp:

09/01/06 09:48:48 (7 years ago)

Author:

simonpj@…

Comment:

--

MonomorphicPatternBindings

@@ -49 +49 @@
 == Experiment ==
 
-In July I changed GHC (the HEAD) to make pattern bindings monomorphic by default.  (A binding of a simple variable is not considered to be a pattern binding.)  The flag {{{-fno-mono-pat-binds}}} restores the standard behaviour.
+In July I changed GHC (the HEAD) to make pattern bindings monomorphic by default.  (A binding of a simple variable is not considered to be a pattern binding.)  The flag {{{-fno-mono-pat-binds}}} restores the standard behaviour.  I deliberately made the new behaviour the default so that I'd hear of any breakage.
 
-The interesting observation is this: all of the libraries compile without a tremor, and I have received no mail whatsoever complaining about the new behaviuor.  I deliberated made the new behaviour the default so that I'd hear of any breakaga. 
+The interesting observation is this: all of the libraries compile without a tremor, and I have received only one message remarking on the new behaviour.  Ross Paterson sent me this code
+{{{
+import Control.Monad.ST
 
-My conclusion: polymorphic pattern bindings is a feature that is virtually never used.  We should nuke them.
+newtype ListMap m a b = ListMap ([a] -> m [b])
 
+runMap :: (forall s. ListMap (ST s) a b) -> [a] -> [b]
+runMap lf as = runST (f as)
+          where ListMap f = lf
+}}}
+This no longer works because {{{f}}} isn't polymorphic, and {{{runST}}} needs it to be.  It's easily fixed:
+{{{
+runMap lf as = runST (f as)
+          where f = case lf of { ListMap f -> f }
+}}}
+My conclusion: polymorphic pattern bindings is a feature that is virtually never used, and not even necessary then.  We should nuke them.
+