From 8a1d397dba3d863752da8f223dc4a3fc0b9cc798 Mon Sep 17 00:00:00 2001
From: Ryan Scott <ryan.gl.scott@gmail.com>
Date: Thu, 20 Jul 2017 14:14:00 -0400
Subject: [PATCH] Emit mustache template warnings (#152)

---
 Criterion/Report.hs | 26 +++++++++++++++++++++++---
 changelog.md        |  6 ++++++
 criterion.cabal     |  2 +-
 3 files changed, 30 insertions(+), 4 deletions(-)

diff --git a/Criterion/Report.hs b/Criterion/Report.hs
index 5da95a04..9b24a4a2 100644
--- a/Criterion/Report.hs
+++ b/Criterion/Report.hs
@@ -28,7 +28,7 @@ module Criterion.Report
     ) where
 
 import Control.Exception (Exception, IOException, throwIO)
-import Control.Monad (mplus)
+import Control.Monad (mplus, unless)
 import Control.Monad.IO.Class (MonadIO(liftIO))
 import Control.Monad.Reader (ask)
 import Criterion.Monad (Criterion)
@@ -42,7 +42,9 @@ import Paths_criterion (getDataFileName)
 import Statistics.Function (minMax)
 import System.Directory (doesFileExist)
 import System.FilePath ((</>), (<.>), isPathSeparator)
-import Text.Microstache (Key (..), Template (..), Node (..), compileMustacheText, renderMustache)
+import System.IO (hPutStrLn, stderr)
+import Text.Microstache (Key (..), MustacheWarning (..), Node (..), Template (..),
+                         compileMustacheText, displayMustacheWarning, renderMustacheW)
 import Prelude ()
 import Prelude.Compat
 import qualified Control.Exception as E
@@ -106,7 +108,25 @@ formatReport reports templateName = do
             , "js-flot"   .= flot
             ]
 
-    return (renderMustache template context)
+    let (warnings, formatted) = renderMustacheW template context
+    -- If there were any issues during mustache template rendering, make sure
+    -- to inform the user. See #127.
+    forM_ warnings $ \warning -> do
+      -- The one thing we choose not to warn about is substituting in the `json`
+      -- key. The reason is that `json` is used in:
+      --
+      --   var reports = {{{json}}};
+      --
+      -- So `json` represents a raw JavaScript array. This is a bit skeevy by
+      -- mustache conventions, but redesigning the template to avoid this
+      -- warning would be more work than just substituting the array directly.
+      unless (warning == MustacheDirectlyRenderedValue (Key ["json"])) $
+        mapM_ (hPutStrLn stderr)
+         [ "criterion: warning:"
+         , "  " ++ displayMustacheWarning warning
+         , ""
+         ]
+    return formatted
   where
     includeTemplate :: (FilePath -> IO T.Text) -> Template -> IO Template
     includeTemplate f Template {..} = fmap
diff --git a/changelog.md b/changelog.md
index 0e313028..d648dd75 100644
--- a/changelog.md
+++ b/changelog.md
@@ -1,3 +1,9 @@
+next
+
+* Have `criterion` emit warnings if suspicious things happen during mustache
+  template substitution when creating HTML reports. This can be useful when
+  using custom templates with the `--template` flag.
+
 1.2.1.0
 
 * Add `GCStatistics`, `getGCStatistics`, and `applyGCStatistics` to
diff --git a/criterion.cabal b/criterion.cabal
index ba2578f6..1beb5cc3 100644
--- a/criterion.cabal
+++ b/criterion.cabal
@@ -94,7 +94,7 @@ library
     exceptions >= 0.8.2 && < 0.9,
     filepath,
     Glob >= 0.7.2,
-    microstache >= 1 && < 1.1,
+    microstache >= 1.0.1 && < 1.1,
     js-flot,
     js-jquery,
     mtl >= 2,