Is there an operator for function composition in Julia?

Question

Say I have two functions:

f(x) = x^2
g(x) = x + 2

Their composition is the function

h(x) = f(g(x))

Is there an operator for function composition in Julia? For example, if * was an operator for function composition (which it isn't), we could write:

h = f * g

P.S. I know I can define it if I want to,

*(f::Function, g::Function) = x -> f(g(x))

Just asking if there is an operator form already in Julia.

Ortrude

Perhaps using the ∘ operator (enter using \circ TAB at REPL) will serve for a better math-y look. – Forebode

@DanGetz Agree, but that is also undefined by default. – Ortrude

The route for getting to default would likely be: Define in your code -> In a package -> In a popular package -> In Base. – Forebode

Answer 1

Coming here later. ∘ is available by default

Spectre

Yes, since github.com/JuliaLang/julia/pull/17155 Julia now has the ∘ operator for function composition. – Ortrude

Answer 2

It is currently an open issue to create such operator, but as now you can keep to the syntax:

julia> h(x) = f(g(x))

or a bit more clearer (for more complex functions):

julia> h(x) = x |> g |> f

It seems as for now you would need to keep the x for making it a composite function.

Another option, is to create your own operator (as you suggest):

julia> ∘(f::Function, g::Function) = x->f(g(x))
julia> h = f ∘ g

This works perfectly fine, however, it introduces a lambda function, and I cannot think a way of performing such operation without lambdas.

NOTE: ∘ operator can be written as \circ as @DanGetz suggested.

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EDIT: seems fast closures are coming in future releases and will probably be easy to implement an efficient version of the composite operator.

Scoter

fast closures are now in Julia 0.5. Any updates on a composition operator? – Ortrude

This is now in Julia 0.6 (as ∘ ) – Herbherbaceous

@Imanol As yours is the accepted answer, perhaps you would consider updating it with a note that \circ is now the composition operator? Thanks. – Neutretto