Dear GHC-ers,

It seems I do not quite understand the behaviour of the inliner. Consider these two modules:

module Foo.Bar where

foo :: Char -> IO ()

foo = putChar

{-# INLINE bar #-}

bar :: String -> IO ()

bar = mapM_ foo

module Main where

import Foo.Bar

main = bar "done"

I would expect the inliner to produce at least (considering the pragma)

main = mapM_ foo "done"

or maybe even

main=mapM_ putChar "done"

However, when I compile this code, that inlining does not seem to happen:

$ ghc -ddump-inlinings -fforce-recomp Main.hs

[1 of 2] Compiling Foo.Bar          ( Foo/Bar.hs, Foo/Bar.o )

Inlining done: Foo.Bar.foo

[2 of 2] Compiling Main             ( Main.hs, Main.o )

Linking Main ...

Indeed, when I use -ddump-simpl, ghc reports:

==================== Tidy Core ====================

Result size of Tidy Core = {terms: 7, types: 5, coercions: 0}

Main.main :: GHC.Types.IO ()

[GblId, Str=DmdType]

Main.main = Foo.Bar.bar (GHC.CString.unpackCString# "done"#)

:Main.main :: GHC.Types.IO ()

[GblId, Str=DmdType]

:Main.main = GHC.TopHandler.runMainIO @ () Main.main

Can anyone explain why Foo.Bar.bar isn’t inlined in Main.main?

Regards,

Philip