Executing code in @eval

General Usage
2

This seems to work:

    @eval struct $sym
        value::String
        function $sym(v::String)
            $(
                if len === nothing
                    nothing
                else
                    :(length(v) == $len || throw(DomainError(v, "")))
                end
            )
            new(v)
        end

julia> sym2("aa")
sym2("aa")

julia> sym2("aaaa")
ERROR: DomainError with aaaa:

Stacktrace:
 [1] sym2(v::String)
   @ Main ./REPL[422]:8
 [2] top-level scope
   @ REPL[431]:1

julia> sym1("aaaa")
sym1("aaaa")

Are you sure this is the best way to handle your use case though? Obviously you are the best judge of that, and perhaps your real use is more complex, but for something like this I’d probably use a parametric type instead of defining types in a for loop

edit: looks like you can also simplify to

@eval struct $sym
    value::String
    function $sym(v::String)
        if $len !== nothing
            length(v) == $len || throw(DomainError(v, ""))
        end
        new(v)
     end
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