# Where is \`isa\` defined

**URL:** <https://discourse.julialang.org/t/where-is-isa-defined/90015>\
**Category:** Internals & Design\
**Created:** [November 10, 2022, 1:18am UTC](https://discourse.julialang.org/t/where-is-isa-defined/90015 "2022-11-10T01:18:30Z")\
**Posts on this page:** 6\
**Page:** 1

<div class="post-metadata">

**Author:** ![Lilith](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lilith/32/27492_2.png) [@Lilith](https://discourse.julialang.org/u/Lilith)\
**Post date:** [November 10, 2022, 1:18am UTC](https://discourse.julialang.org/t/where-is-isa-defined/90015/1 "2022-11-10T01:18:30Z")

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```julia
julia> @less 1 isa Int
ERROR: could not determine location of method definition

```

How else can I locate the method definition? Grepping for `isa` is tricky.

---

<div class="post-metadata">

**Author:** ![jar1](https://avatars.discourse-cdn.com/v4/letter/j/c0e974/32.png) [@jar1](https://discourse.julialang.org/u/jar1)\
**Post date:** [November 10, 2022, 1:19am UTC](https://discourse.julialang.org/t/where-is-isa-defined/90015/2 "2022-11-10T01:19:36Z")

</div>

```julia

julia> methods(isa)
# 1 method for builtin function "isa":
[1] isa(...) in Core

```

---

<div class="post-metadata">

**Author:** ![CameronBieganek](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/cameronbieganek/32/6915_2.png) [@CameronBieganek](https://discourse.julialang.org/u/CameronBieganek)\
**Post date:** [November 10, 2022, 4:42am UTC](https://discourse.julialang.org/t/where-is-isa-defined/90015/3 "2022-11-10T04:42:17Z")

</div>

Built-in functions print `(built-in function)` if you evaluate them (just their name) in the REPL:

```nohighlight
julia> isa
isa (built-in function)

julia> getfield
getfield (built-in function)

```

---

<div class="post-metadata">

**Author:** ![Lilith](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lilith/32/27492_2.png) [@Lilith](https://discourse.julialang.org/u/Lilith)\
**Post date:** [November 10, 2022, 6:01am UTC](https://discourse.julialang.org/t/where-is-isa-defined/90015/4 "2022-11-10T06:01:47Z")

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Aye, but where is this built-in function defined?

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<div class="post-metadata">

**Author:** ![uniment](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/uniment/32/24532_2.png) [@uniment](https://discourse.julialang.org/u/uniment)\
**Post date:** [November 10, 2022, 7:01am UTC](https://discourse.julialang.org/t/where-is-isa-defined/90015/5 "2022-11-10T07:01:42Z")

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Bermuda triangle.

```julia
julia> methods(isa)[1].file
Symbol("")

julia> methods(isa)[1].line
0

```

---

<div class="post-metadata">

**Author:** ![kristoffer.carlsson](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/kristoffer.carlsson/32/22_2.png) [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)\
**Post date:** [November 10, 2022, 7:24am UTC](https://discourse.julialang.org/t/where-is-isa-defined/90015/6 "2022-11-10T07:24:20Z")

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> <https://github.com/JuliaLang/julia/blob/0a65af86ad2b14d28c85d3afce161cdf24a93f66/src/builtins.c#L1940>

> <https://github.com/JuliaLang/julia/blob/0a65af86ad2b14d28c85d3afce161cdf24a93f66/src/builtins.c#L512-L517>

> <https://github.com/JuliaLang/julia/blob/0a65af86ad2b14d28c85d3afce161cdf24a93f66/src/subtype.c#L2073-L2123>