package ppx_compose

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Inlined function composition


Dune Dependency





ppx_compose - Inlined Function Composition

ppx_compose is a simple syntax extension which rewrites code containing function compositions into composition-free code, effectively inlining the composition operators. The following two operators are supported

let (%) g f x = g (f x)
let (%>) f g x = g (f x)

Corresponding definitions are not provided, so partial applications of (%) and (%>) will be undefined unless you provide the definitions.

The following rewrites are done:

  • A composition occurring to the left of an application is reduced by applying each term of the composition from right to left to the argument, ignoring associative variations.

  • A composition which is not the left side of an application is first turned into one by η-expansion, then the above rule applies.

  • Any partially applied composition operators are passed though unchanged.


h % g % f ==> (fun x -> h (f (g x)))
h % (g % f) ==> (fun x -> h (f (g x)))
(g % f) (h % h) ==> g (f (fun x -> h (h x)))

Is It Needed?

Recent flambda-enabled compilers can inline the following alternative definitions of the composition operators [1]:

let (%) g f = (); fun x -> g (f x)
let (%>) f g = (); fun x -> g (f x)

so this syntax extension will likely be retired at some point.


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