[yesod-bin] improve stack detection for 'stack keter'

The following will now use stack:

* `stack query` succeeds from current directory instead of searching that a `stack.yaml` exists
* `STACK_YAML` or `STACK_EXE`(set by `stack exec`) environment variables are set

yesod-bin/Keter.hs

@@ -6,11 +6,12 @@ module Keter
 import Data.Yaml
 import qualified Data.HashMap.Strict as Map
 import qualified Data.Text as T
+import System.Environment (getEnvironment)
 import System.Exit
 import System.Process
 import Control.Monad
 import System.Directory hiding (findFiles)
-import Data.Maybe (mapMaybe)
+import Data.Maybe (mapMaybe,isJust,maybeToList)
 import Data.Monoid
 import System.FilePath ((</>))
 import qualified Codec.Archive.Tar as Tar
@@ -48,6 +49,8 @@ keter cabal noBuild noCopyTo buildArgs = do
                             _ -> return value
             Just _ -> error $ ketercfg ++ " is not an object"
 
+    env' <- getEnvironment
+    cwd' <- getCurrentDirectory
     files <- getDirectoryContents "."
     project <-
         case mapMaybe (T.stripSuffix ".cabal" . T.pack) files of
@@ -74,11 +77,22 @@ keter cabal noBuild noCopyTo buildArgs = do
         collapse' (x:xs) = x : collapse' xs
         collapse' [] = []
 
-    unless noBuild $ if elem "stack.yaml" files
-        then do run "stack" ["clean"]
-                createDirectoryIfMissing True "./dist/bin"
+    stackQueryRunSuccess <- do
+      (ec,_,_) <- readProcessWithExitCode "stack" ["query"] ""
+      return (ec == ExitSuccess)
+
+    let inStackExec = isJust $ lookup "STACK_EXE" env'
+        mStackYaml = lookup "STACK_YAML" env'
+        useStack = inStackExec || isJust mStackYaml || stackQueryRunSuccess
+
+    unless noBuild $ if useStack
+        then do let stackYaml = maybeToList $ fmap ("--stack-yaml="<>) mStackYaml
+                    localBinPath = cwd' </> "dist/bin"
+                run "stack" $ stackYaml <> ["clean"]
+                createDirectoryIfMissing True localBinPath
                 run "stack"
-                    ((words "--local-bin-path ./dist/bin build --copy-bins")
+                    (stackYaml
+                     <> ["--local-bin-path",localBinPath,"build","--copy-bins"]
                      <> buildArgs)
         else do run cabal ["clean"]
                 run cabal ["configure"]