Dynamic struct keys not allowed: "error: invalid key field for struct, struct keys must be atoms"

[azizk]

Existing issue

• I have searched existing issues and could not find a duplicate.

Elixir and Erlang/OTP versions

Erlang/OTP 29 [erts-17.0.6] [source] [64-bit] [smp:16:16] [ds:16:16:10] [async-threads:1] [jit:ns]

Elixir 1.20.4 (compiled with Erlang/OTP 29)

Operating system

NixOS

Current behavior

Dynamic key variables are not allowed in a struct:

field = :path
%URI{field => "/"}

And always produce the following error:

error: invalid key field for struct, struct keys must be atoms, got: 
└─ iex:1

** (CompileError) cannot compile code (errors have been logged)

Expected behavior

This here on the other hand just works:

field = :path
%{%URI{} | field => "/"}

Would be good if it worked directly.

[josevalim]

Yes, this is by definition. All struct keys are checked at compile time. This is not a bug and is expected behavior.

[sabiwara]

@josevalim I think we could still improve the error message in this case though, right now it is confusing and cryptic:

error: invalid key field for struct, struct keys must be atoms, got:
└─ iex:41: A.a/0

** (KeyError) key {:field, [version: 0, line: 41], nil} not found
    (elixir 1.20.3) expanding struct: URI.__struct__/1
    iex:41: (file)

[josevalim]

Yes