GitLab

Consider

module Lib where

import Control.Monad.ST

f :: IO ()
f = return ()
{-# NOINLINE f #-}

g :: ST s ()
g = return ()
{-# NOINLINE g #-}

Compiled with -O2, we get

Lib.g1
  :: forall s. GHC.Prim.State# s -> (# GHC.Prim.State# s, () #)
Lib.g1
  = \ (@ s_a1oA) (s1_X1qD :: GHC.Prim.State# s_a1oA) ->
      (# s1_X1qD, GHC.Tuple.() #)

g [InlPrag=NOINLINE] :: forall s. ST s ()
g = Lib.g1
    `cast` (forall (s :: <*>_N). Sym (GHC.ST.N:ST[0] <s>_N <()>_R)
            :: (forall s. GHC.ST.STRep s ()) ~R# (forall s. ST s ()))

Lib.f1
  :: GHC.Prim.State# RealWorld -> (# GHC.Prim.State# RealWorld, () #)
Lib.f1
  = \ (s_a1r4 :: GHC.Prim.State# RealWorld) ->
      (# s_a1r4, GHC.Tuple.() #)

f [InlPrag=NOINLINE] :: IO ()
f = Lib.f1
    `cast` (Sym (GHC.Types.N:IO[0] <()>_R)
            :: (GHC.Prim.State# RealWorld
                -> (# GHC.Prim.State# RealWorld, () #))
               ~R# IO ())

Note that f and g (the wrappers) are marked NOINLINE instead of Lib.f1 and Lib.g1 (the workers wrt. casts/eta expansion), respectively. This might not look harmful in itself, but it inhibits NOINLINE function in IO and ST from exposing wrappers as a result from strictness analysis (which is useful, #13143 (closed)), as the following example demonstrates:

module Lib where

h :: Int -> IO ()
h x = do
  x `seq` return ()
  print "foo"
  print "foo"
  print "foo"
{-# NOINLINE h #-}

Lib.$wh
  :: Int
     -> GHC.Prim.State# RealWorld -> (# GHC.Prim.State# RealWorld, () #)
Lib.$wh = <elided>

Lib.h1
  :: Int
     -> GHC.Prim.State# RealWorld -> (# GHC.Prim.State# RealWorld, () #)
Lib.h1
  = \ (w_s1MP :: Int) (w1_s1MQ :: GHC.Prim.State# RealWorld) ->
      case w_s1MP of w2_X1Ni { GHC.Types.I# ipv_s1MX ->
      Lib.$wh w2_X1Ni w1_s1MQ
      }

h [InlPrag=NOINLINE] :: Int -> IO ()
h = Lib.h1
    `cast` (<Int>_R ->_R Sym (GHC.Types.N:IO[0] <()>_R)
            :: (Int
                -> GHC.Prim.State# RealWorld
                -> (# GHC.Prim.State# RealWorld, () #))
               ~R# (Int -> IO ()))

in #13143 (closed) we decided to mark the worker as NOINLINE, so we should do the same for eta expansion.