From 3891f80bf570ddf2882d580de54f66e3af5454bb Mon Sep 17 00:00:00 2001
From: Michael Peyton Jones <me@michaelpj.com>
Date: Thu, 23 May 2019 19:00:35 +0100
Subject: [PATCH] Abort more gracefully if the source files don't exist

Fixes #59.
---
 app/Niv.hs | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/app/Niv.hs b/app/Niv.hs
index 7b155c0..0959349 100644
--- a/app/Niv.hs
+++ b/app/Niv.hs
@@ -58,6 +58,9 @@ newtype Sources = Sources
 
 getSources :: IO Sources
 getSources = do
+    exists <- Dir.doesFileExist pathNixSourcesJson
+    unless exists abortSourcesDoesntExist
+
     warnIfOutdated
     -- TODO: if doesn't exist: run niv init
     putStrLn $ "Reading sources file"
@@ -696,6 +699,14 @@ Make sure the repository exists.
 -- Abort
 -------------------------------------------------------------------------------
 
+abortSourcesDoesntExist :: IO a
+abortSourcesDoesntExist = abort $ unlines [ line1, line2 ]
+  where
+    line1 = "Cannot use " <> pathNixSourcesJson
+    line2 = [s|
+The sources file does not exist! You may need to run 'niv init'.
+|]
+
 abortSourcesIsntAMap :: IO a
 abortSourcesIsntAMap = abort $ unlines [ line1, line2 ]
   where