'Int' as a reserved word

My code has such a line


It got an error:

ERROR: cannot assign a value to imported variable Core.Int from module Main

It seems that ‘Int’ is a reserved word of julia. How can I check whether a variable name is reserved or not?

I mean, you’ve already experienced one way that you can check: if you try to assign to a variable name and get the error you just got, that name is unavailable (in current module scope). Beyond that, the answer to your question somewhat depends on why you want to do this:

  1. Are you wanting to programmatically find out what names are available for assignment for the purpose of e.g. writing some developer tooling?
  2. Do you just want to be able to use the names you want even if they’re defined elsewhere?
  3. Do you not want your long-running calculation to error out after 30 minutes because you have a Int=5 statement somewhere in your code, so you want advance notice if you ever make this error?

For 1, doing this programmatically isn’t necessarily as easy as it seems like it should be. Basically what you’re asking is to get a list of all symbols defined in a specific module. Theoretically, I think this is what the names function is intended to be for, but practically it doesn’t get you there. The best answer I’ve seen is from Tim Holy a few years ago:

function namesin(mymod)
    nms = names(mymod; imported=true)
    for mod in Base.loaded_modules_array()
        if isdefined(mymod, nameof(mod))
            append!(nms, names(mod))

And then you could use this funciton to check whether e.g. Int is already assigned:

julia> :Int in namesin(Main)

julia> :other_symbol in namesin(Main)

For 2, you do have options: you can create a module without Int defined within it, or you can use let to shadow variable names within the let-block’s scope:

julia> baremodule ThisModuleOnlyImportsCore
           Int = 5

julia> ThisModuleOnlyImportsCore.Int

julia> let Int=6
           x = Int + 3
           println("let statements always assign to a new variable location instead of trying to assign to existing")
let statements always assign to a new variable location instead of trying to assign to existing

For 3, I believe the linting capability of LanguageServer.jl (used in e.g. the julia vs-code extension) should catch this, but don’t quote me on that.


To clarify, you can assign to the variable Int, as long as you haven’t already used the Int that was imported from Core. Try this in a fresh Julia session:

julia> Int = 1

The issue you illustrated only happens if you’ve already used Int from Core before you try to assign to it. The same thing happens with any other variables exported from a module, as you can see in this example:

julia> module A
       x = 1
       export x

julia> using .A

julia> x

julia> x = 10
ERROR: cannot assign a value to imported variable A.x from module Main

Just to state this explicitly: Int is not a reserved name in Julia. You can find a list of reserved names in the Manual, here: Keywords.