GHC: Inline Pragma

gistfile1.txt

## INLINEABLE vs INLINE

INLINEABLE is prefered. (Source: <https://wiki.haskell.org/Inlining_and_Specialisation>

inline-pragma.md

{-# INLINE ⟨name⟩ #-}

Source: https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts.html#inline-pragma

key_function :: Int -> String -> (Bool, Double)
{-# INLINE key_function #-}

Pointless inline are not applied

map key_function xs
# key_function is not inlined because it's the same as calling the function
map (\x -> body) xs

Only fully applied functions are inlined!

comp1 :: (b -> c) -> (a -> b) -> a -> c
{-# INLINE comp1 #-}
comp1 f g = \x -> f (g x)

comp2 :: (b -> c) -> (a -> b) -> a -> c
{-# INLINE comp2 #-}
comp2 f g x = f (g x)

# INLINED
map (not `comp1` not) xs

# NOT
map (not `comp2` not) xs

We don't optimize before inline (otherwise rewrite rules may not be applied). GHC guarantees to inline precisely the code that you wrote.

{-# INLINABLE ⟨name⟩ #-}

"Feel free to inline me".

That decision is made at the call site, and will therefore be affected by the inlining threshold, optimisation level etc.

{-# NOINLINE ⟨name⟩ #-}

Phase Control

Inlining happens only during runs of the simplifier. Each run of the simplifier has a different phase number; the phase number decreases towards zero.

• INLINE[k] f means: do not inline f until phase k, but from phase k onwards be very keen to inline it.

• INLINE[~k] f means: be very keen to inline f until phase k, but from phase k onwards do not inline it.

The same phase-numbering control is available for RULEs (Rewrite rules).

You usually want to INLINE before REWRITE.