From b35eed2a9f1c82131f51f55c771ac2372127520d Mon Sep 17 00:00:00 2001
From: Alex Biehl <alexbiehl@gmail.com>
Date: Fri, 12 May 2017 21:02:33 +0200
Subject: Haddock: Fix broken lazy IO in prologue reading (#615)

We previously used withFile in conjunction with hGetContents. The list returned
by the latter wasn't completely forced by the time we left the withFile block,
meaning that we would try to read from a closed handle.
---
 haddock-api/src/Haddock.hs | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/haddock-api/src/Haddock.hs b/haddock-api/src/Haddock.hs
index 3971a5b7..f0e7e6c7 100644
--- a/haddock-api/src/Haddock.hs
+++ b/haddock-api/src/Haddock.hs
@@ -547,9 +547,10 @@ getPrologue :: DynFlags -> [Flag] -> IO (Maybe (MDoc RdrName))
 getPrologue dflags flags =
   case [filename | Flag_Prologue filename <- flags ] of
     [] -> return Nothing
-    [filename] -> withFile filename ReadMode $ \h -> do
+    [filename] -> do
+      h <- openFile filename ReadMode
       hSetEncoding h utf8
-      str <- hGetContents h
+      str <- hGetContents h -- semi-closes the handle
       return . Just $! parseParas dflags str
     _ -> throwE "multiple -p/--prologue options"
 
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