From 328176735bf68e1e6523648dee04b6af73d2bee2 Mon Sep 17 00:00:00 2001
From: Ken Micklas <kmicklas@gmail.com>
Date: Sat, 20 Oct 2018 12:23:08 -0400
Subject: [PATCH] Add fallbackCompletion to support module completion in ghci
 :add (#91)

---
 System/Console/Haskeline/Completion.hs | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/System/Console/Haskeline/Completion.hs b/System/Console/Haskeline/Completion.hs
index b17bb0c7..04b8e9a9 100644
--- a/System/Console/Haskeline/Completion.hs
+++ b/System/Console/Haskeline/Completion.hs
@@ -3,6 +3,7 @@ module System.Console.Haskeline.Completion(
                             Completion(..),
                             noCompletion,
                             simpleCompletion,
+                            fallbackCompletion,
                             -- * Word completion
                             completeWord,
                             completeWordWithPrev,
@@ -188,3 +189,12 @@ fixPath ('~':c:path) | isPathSeparator c = do
     home <- getHomeDirectory
     return (home </> path)
 fixPath path = return path
+
+-- | If the first completer produces no suggestions, fallback to the second
+-- completer's output.
+fallbackCompletion :: Monad m => CompletionFunc m -> CompletionFunc m -> CompletionFunc m
+fallbackCompletion a b input = do
+    aCompletions <- a input
+    if null (snd aCompletions)
+        then b input
+        else return aCompletions