# What is \`Callable\`

**URL:** <https://discourse.julialang.org/t/what-is-callable/73884>\
**Category:** Internals & Design\
**Created:** [December 31, 2021, 11:39pm UTC](https://discourse.julialang.org/t/what-is-callable/73884 "2021-12-31T23:39:34Z")\
**Posts on this page:** 1\
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**Author:** ![mikmoore](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mikmoore/32/31109_2.png) [@mikmoore](https://discourse.julialang.org/u/mikmoore)\
**Post date:** [January 1, 2022, 12:41am UTC](https://discourse.julialang.org/t/what-is-callable/73884/2 "2022-01-01T00:41:34Z")

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See discussion in [#43491](https://github.com/JuliaLang/julia/issues/43491). The short answer is that it’s `Union{Function,Type}`, like you’d guessed, but that you really should avoid using it unless ambiguities would arise otherwise. If we were better people, we probably would have already removed `::Callable` from existence. But we haven’t gotten around to it yet.

Since the body of the function would presumably include `default(key)`, I think the function will choose to specialize on `typeof(default)` whether or not you use the `::T` annotation. But I could be mistaken. You can try it with and without to see if it makes a difference. Be sure to restart julia before finalizing your decision, because sometimes the performance doesn’t update properly in this case from `Revise` alone.

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