Expr does not have object identity have field equality, i.e. julia> Expr(:call, println) == Expr(:call, println) true I was surprised to learn that Expr nevertheless is mutable. I can easily imagine why Expr does not have object identity (“Oh, no, I do not mean to call this println method without …

[image] PatrickHaecker: So why is Expr mutable? One reason might be that Expr existed long before immutable structs were implemented in the language.

=== is the comparison for object identity, and it’ll be false for your first example.

[image] Benny: === is the comparison for object identity, and it’ll be false for your first example. Thanks for the correction! Do you know of a better term for == being true although === might be false?

julia> a = [1,2,3]; b=[1,2,3]; julia> a == b true julia> a === b false julia> a = 1; b =1 1 julia> a == b true julia> a === b true julia> struct Foo x::Int64 end julia> a = Foo(1); b = Foo(1); julia> a == b true julia> a === b true julia> mutable struct Goo x::Int64 en…

If a === b we say that a and b are “egal”, whereas if a == b we say they are “equal”. Egality is a “stronger” condition

[image] Mason: if a == b we say they are “equal” Thanks to all the suggestions. I needed it as a noun and changed the post to use field equality. Sorry for the confusion with the terminology. Back to the original question: Does anyone know the reason why Expr is mutable? I think @GunnarFarne…

I can only guess, but my inkling is that it is some combination of one or more of the following: Having Expr immutable doesn’t provide much gains in practice. Many existing macros would break from the change. While args is indeed mutable, it’s nevertheless easier to be able to replace it than havi…

[image] GunnarFarneback: I can only guess, but my inkling is that it is some combination of one or more of the following: My guess is that it is mostly historical: the Expr type dates to the earliest days of Julia, before immutable struct types existed in the language, and since they aren’t …

Why is `Expr` mutable?

Internals & Design

Topic		Replies	Views
How is `Symbol` special? Internals & Design	8	359	June 14, 2025
Assignment and argument passing semantics Offtopic	31	2972	November 4, 2018
A metaprogramming question (sorry, can't find a more informative title) New to Julia	9	937	February 2, 2017
Default `==` for struct and mutability General Usage question	2	241	December 15, 2022
Equality of mutable structs General Usage mutable-structure	7	666	December 5, 2023

Why is `Expr` mutable?

Related topics