Try to set up error handling for GitHub Commit plugin

This doesn't really work, but I wanted to hold onto this step. I've been
reading about error handling, notably this article:

http://www.randomhacks.net/2007/03/10/haskell-8-ways-to-report-errors/

I kind of like the idea of just using `fail`, and being able to have
that set up some polymorphism to handle `Maybe`, `Either`, and `IO`
situations, but today read some literature that discouraged using fail,
since not all monads implement it.

Anyway, this code doesn't print out the error string like I intended it
to, so I guess I'll have to use `Either` instead.

1 files changed, 7 insertions, 4 deletions

diff --git a/src/Plugin/GitHubCommit.hs b/src/Plugin/GitHubCommit.hs
index cac74a4..8c11a12 100644
--- a/src/Plugin/GitHubCommit.hs
+++ b/src/Plugin/GitHubCommit.hs
@@ -24,12 +24,15 @@ gitHubCommitAction message dbConn = do
         \ LIMIT 1"
         (Only (M.channel message))
         :: IO [RepoUrlRow]
-    return $ response rs
+    -- return $ response rs
+    respond rs
   where
-    response []     = ""
-    response ((RepoUrlRow r):rs) =
-        r ++ "/commits/" ++ M.text message =~ matchRegex gitHubCommit
+    respond []     = fail "I couldn't find a repo URL for this channel. \
+        \ Try `git remote set origin REPO_URL`"
+    respond ((RepoUrlRow r):rs) =
+        return r ++ "/commits/" ++ M.text message =~ matchRegex gitHubCommit
 -- TODO: Make an Either type for plugins to return errors
+-- if empty query result, return an empty error, otherwise return the string
 
 type Id = Int