From ffdf4af243efdaf8f75147012d9348a4c7c4a07b Mon Sep 17 00:00:00 2001
From: Chris Done <chrisdone@gmail.com>
Date: Fri, 22 May 2015 11:32:57 +0200
Subject: [PATCH] Generalize stripDir to MonadThrow

---
 src/Path.hs | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/src/Path.hs b/src/Path.hs
index 3951980..e7e8960 100644
--- a/src/Path.hs
+++ b/src/Path.hs
@@ -239,9 +239,14 @@ toFilePath (Path l) = l
 --
 -- In other words the bases must match.
 --
-stripDir :: Path b Dir -> Path b t -> Maybe (Path Rel t)
+-- Throws: 'Couldn'tStripPrefixDir'
+--
+stripDir :: MonadThrow m
+         => Path b Dir -> Path b t -> m (Path Rel t)
 stripDir (Path p) (Path l) =
-  fmap Path (stripPrefix p l)
+  case fmap Path (stripPrefix p l) of
+    Nothing -> throwM (Couldn'tStripPrefixDir p l)
+    Just ok -> return ok
 
 -- | Is p a parent of the given location? Implemented in terms of
 -- 'stripDir'. The bases must match.