One-Shotify GHC.Utils.Monad.State (#18202)

1 files changed, 15 insertions, 1 deletions

diff --git a/compiler/GHC/Utils/Monad/State.hs b/compiler/GHC/Utils/Monad/State.hs
index c7b9e3f591..997137525b 100644
--- a/compiler/GHC/Utils/Monad/State.hs
+++ b/compiler/GHC/Utils/Monad/State.hs
@@ -1,13 +1,27 @@
 {-# LANGUAGE DeriveFunctor #-}
 {-# LANGUAGE UnboxedTuples #-}
+{-# LANGUAGE PatternSynonyms #-}
 
 module GHC.Utils.Monad.State where
 
 import GHC.Prelude
 
-newtype State s a = State { runState' :: s -> (# a, s #) }
+import GHC.Exts (oneShot)
+
+newtype State s a = State' { runState' :: s -> (# a, s #) }
     deriving (Functor)
 
+pattern State :: (s -> (# a, s #))
+              -> State s a
+
+-- This pattern synonym makes the monad eta-expand,
+-- which as a very beneficial effect on compiler performance
+-- See #18202.
+-- See Note [The one-shot state monad trick] in GHC.Utils.Monad
+pattern State m <- State' m
+  where
+    State m = State' (oneShot $ \s -> m s)
+
 instance Applicative (State s) where
    pure x   = State $ \s -> (# x, s #)
    m <*> n  = State $ \s -> case runState' m s of