# A precise definition of what it means to be an instance of an abstract type

**URL:** <https://discourse.julialang.org/t/a-precise-definition-of-what-it-means-to-be-an-instance-of-an-abstract-type/57270>\
**Category:** New to Julia\
**Tags:** question\
**Created:** [March 16, 2021, 7:45am UTC](https://discourse.julialang.org/t/a-precise-definition-of-what-it-means-to-be-an-instance-of-an-abstract-type/57270 "2021-03-16T07:45:56Z")\
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**Author:** ![path-doc](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/path-doc/32/19183_2.png) [@path-doc](https://discourse.julialang.org/u/path-doc)\
**Post date:** [March 17, 2021, 12:41pm UTC](https://discourse.julialang.org/t/a-precise-definition-of-what-it-means-to-be-an-instance-of-an-abstract-type/57270/36 "2021-03-17T12:41:13Z")

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**Definition** : Object x is an _instance_ of the abstract type y if, ~~and only if~~ ,

1. the concrete type t\_x of x and the value v\_y of y satisfy t\_x\<:v\_y, or
2. y is a singleton type and x is its instance (which can be verified using `isa`)
3. …  
[What is difference between Type{T} and T - #58 by path-doc](https://discourse.julialang.org/t/what-is-difference-between-type-t-and-t/19325/58)

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