[commit: ghc] wip/T9858-typeable-ben: Unify: Add runUM for debugging purposes (ef0a4f7)

[git at git.haskell.org]

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

On branch  : wip/T9858-typeable-ben
Link       : http://ghc.haskell.org/trac/ghc/changeset/ef0a4f7203af38feb355e2f3f60f8ede029fce29/ghc

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

commit ef0a4f7203af38feb355e2f3f60f8ede029fce29
Author: Ben Gamari <ben at smart-cactus.org>
Date:   Wed Oct 28 11:42:09 2015 +0100

    Unify: Add runUM for debugging purposes


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

ef0a4f7203af38feb355e2f3f60f8ede029fce29
 compiler/types/Unify.hs | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/compiler/types/Unify.hs b/compiler/types/Unify.hs
index 7f28bed..2ee3758 100644
--- a/compiler/types/Unify.hs
+++ b/compiler/types/Unify.hs
@@ -733,6 +733,13 @@ instance Monad UM where
 initUM :: (TyVar -> BindFlag) -> UM () -> UnifyResult
 initUM badtvs um = fmap (niFixTvSubst . snd) $ unUM um badtvs emptyTvSubstEnv
 
+-- | Run a 'UM' action without affecting the substition environment, returning the
+-- 'UnifyResultM' that the action concludes with.
+--
+-- For debugging purposes to peer into the decisions that the typechecker makes
+runUM :: UM a -> UM (UnifyResultM (a, TvSubstEnv))
+runUM um = UM $ \tv_fn subst -> Unifiable $ (unUM um tv_fn subst, subst)
+
 tvBindFlag :: TyVar -> UM BindFlag
 tvBindFlag tv = UM (\tv_fn subst -> Unifiable (tv_fn tv, subst))