The Meaning of `const`

[Leah Hanson]

The only reference I can find in the manual to the `const` keyword is in the performance section [1]. Well, that and the `isconst` function[2].

This seems like it would be worth documenting, especially since it behaves significantly differently than the const that I remember from Java or C++, which prevented you from changing the value of const variable.

I think I understand now, that `const` in Julia means that the type of the variable must remain constant. This was very surprising to me.

~~~.jl

julia> const x = 10

10

julia> x += 1

Warning: redefining constant x

11

julia> x

11

julia> const arr = Int[1,2,3]

3-element Array{Int64,1}:

 1

 2

 3

julia> arr[1] = 13

13

julia> arr

3-element Array{Int64,1}:

 13

  2

  3

julia> x += 0.5

ERROR: invalid redefinition of constant x

~~~

I'd be happy to add this to the manual, but I'm not sure where it belongs. Any suggestions?

1: http://docs.julialang.org/en/latest/manual/performance-tips/?highlight=const#avoid-global-variables

2: http://docs.julialang.org/en/latest/stdlib/base/?highlight=const#Base.isconst

[Tim Holy]

Interesting question. Perhaps an example like this:

julia> const x = 10
10

julia> x = 11

Warning: redefining constant x
11

julia> x = 11.0

ERROR: invalid redefinition of constant x

could come fairly early (maybe in "Variables"). I'd say mention of the array
behavior could come in "Multi-dimensional Arrays".

The same issue applies to Dicts, and we don't really talk about them in the
main manual at all.

--Tim

[Mauro]

You missed this: http://docs.julialang.org/en/latest/manual/variables-and-scoping/#constants

However, it is not very clear, in particular concerning constant type vs constant value.  So an update would be good.  Also, does it belong into the scope-section?

[Steven G. Johnson]

On Tuesday, July 8, 2014 11:34:10 PM UTC-4, Leah Hanson wrote:

I think I understand now, that `const` in Julia means that the type of the variable must remain constant. This was very surprising to me.

`const` in Julia is supposed to mean that the symbol is always bound to the same variable, I think.  Technically, you can assign it to another variable of the same type, but you get a warning as you saw above, and you probably shouldn't rely on this being allowed at all (e.g. Julia is free to inline the value of const variables when it compiles a function, so it may not notice if you subsequently change the const value.)  However, if the variable is a mutable type (e.g. Array, Dict, ...), then you can always change the contents of that variable.

So, for example,

       const foo = 3

is much like

       const int foo = 3;

in C: you are not supposed to change the value of foo from 3.

On the other hand,

       const foo = [4,7,1]

is much like

       int * const foo = (int*) malloc(sizeof(int) * 3);

       foo[0] = 4; foo[1] = 7; foo[2] = 1;

in C: foo is a constant pointer to non-constant data.   You can't change foo to point to a different array, but you can change the contents of the array that foo points to.