Format files in parallel

There is no explicit configuration for this, but it is possible for
users to override by changing the RTS options to the `ormolu`
executable.

CHANGELOG.md

@@ -1,3 +1,7 @@
+## Unreleased
+
+* Format multiple files in parallel. [Issue 1128](https://github.com/tweag/ormolu/issues/1128).
+
 ## Ormolu 0.7.7.0
 
 * Use single-line layout for parens around single-line content. [Issue

app/Main.hs

@@ -7,6 +7,7 @@
 
 module Main (main) where
 
+import Control.Concurrent.Async (mapConcurrently)
 import Control.Exception (throwIO)
 import Control.Monad
 import Data.Bool (bool)
@@ -53,7 +54,8 @@ main = do
             ExitSuccess -> Nothing
             ExitFailure n -> Just n
       errorCodes <-
-        mapMaybe selectFailure <$> mapM (formatOne' . Just) (sort xs)
+        -- Format files in parallel, solves #1128
+        mapMaybe selectFailure <$> mapConcurrently (formatOne' . Just) (sort xs)
       return $
         if null errorCodes
           then ExitSuccess

ormolu.cabal

@@ -136,6 +136,7 @@ executable ormolu
     default-language: GHC2021
     build-depends:
         Cabal-syntax >=3.12 && <3.13,
+        async >=2.2 && <2.3,
         base >=4.12 && <5,
         containers >=0.5 && <0.8,
         directory ^>=1.3,
@@ -146,13 +147,17 @@ executable ormolu
         text >=2.1 && <3,
         th-env >=0.1.1 && <0.2
 
+    -- We use parallelism so we need a threaded runtime to get any
+    -- benefit.
+    ghc-options: -threaded -rtsopts -with-rtsopts=-N
+
     if flag(dev)
         ghc-options:
             -Wall -Werror -Wredundant-constraints -Wpartial-fields
             -Wunused-packages -Wwarn=unused-packages
 
     else
-        ghc-options: -O2 -Wall -rtsopts
+        ghc-options: -O2 -Wall
 
 test-suite tests
     type:               exitcode-stdio-1.0