Faster compiled scripts

Note the changelog entry, which explains a downside to this approach.

ChangeLog.md

@@ -98,6 +98,12 @@ Other enhancements:
   [#4535](https://github.com/commercialhaskell/stack/issues/4535)/
 * Show snapshot being used when `stack ghci` is invoked outside of a project directory. See
   [#3651](https://github.com/commercialhaskell/stack/issues/3651)
+* When using the script interpreter with `--optimize` or `--compile`,
+  Stack will perform an optimization of checking whether a newer
+  executable exists, making reruns significantly faster. There's a
+  downside to this, however: if you have a multifile script, and
+  change one of the dependency modules, Stack will not automatically
+  detect and recompile.
 
 Bug fixes:
 

src/Stack/Runners.hs

@@ -22,6 +22,7 @@ module Stack.Runners
     , loadCompilerVersion
     , withUserFileLock
     , munlockFile
+    , withRunnerGlobal
     ) where
 
 import           Stack.Prelude

src/Stack/Script.hs

@@ -28,6 +28,7 @@ import           Stack.Types.Compiler
 import           Stack.Types.Config
 import           Stack.Types.SourceMap
 import           System.FilePath            (dropExtension, replaceExtension)
+import qualified RIO.Directory as Dir
 import           RIO.Process
 import qualified RIO.Text as T
 
@@ -42,6 +43,25 @@ scriptCmd opts go' = do
                 }
             , globalStackYaml = SYLNoConfig scriptDir
             }
+
+    -- Optimization: if we're compiling, and the executable is newer
+    -- than the source file, run it immediately.
+    case soCompile opts of
+      SEInterpret -> longWay file scriptDir go
+      SECompile -> shortCut file scriptDir go
+      SEOptimize -> shortCut file scriptDir go
+
+  where
+  shortCut file scriptDir go = handleIO (const $ longWay file scriptDir go) $ do
+    srcMod <- getModificationTime file
+    exeMod <- Dir.getModificationTime $ toExeName $ toFilePath file
+    if srcMod < exeMod
+      then withRunnerGlobal go' $ \runner ->
+           runRIO runner $
+           exec (toExeName $ toFilePath file) (soArgs opts)
+      else longWay file scriptDir go
+
+  longWay file scriptDir go = do
     withDefaultBuildConfigAndLock go $ \lk -> do
       -- Some warnings in case the user somehow tries to set a
       -- stack.yaml location. Note that in this functions we use
@@ -121,16 +141,16 @@ scriptCmd opts go' = do
               (ghcArgs ++ [toFilePath file])
               (void . readProcessStdout_)
             exec (toExeName $ toFilePath file) (soArgs opts)
-  where
-    toPackageName = reverse . drop 1 . dropWhile (/= '-') . reverse
 
-    -- Like words, but splits on both commas and spaces
-    wordsComma = splitWhen (\c -> c == ' ' || c == ',')
+  toPackageName = reverse . drop 1 . dropWhile (/= '-') . reverse
+
+  -- Like words, but splits on both commas and spaces
+  wordsComma = splitWhen (\c -> c == ' ' || c == ',')
 
-    toExeName fp =
-      if osIsWindows
-        then replaceExtension fp "exe"
-        else dropExtension fp
+  toExeName fp =
+    if osIsWindows
+      then replaceExtension fp "exe"
+      else dropExtension fp
 
 getPackagesFromModuleInfo
   :: ModuleInfo