StackOverflow while defining methods

[baggepinnen]

On julia v1.5.0-rc1

I'm trying to define a bunch of methods using metaprogramming and I'm seeing a stack overflow error upon call to eval.

The following code manages to define the first method in the loop, but while evaling the second one the error is thrown

struct MyType{T,N}
    x::T
end

f(x::MyType) = x.x
f(x) = x

for XT in (:T, Symbol("MyType{T,N}")), YT in (:T, Symbol("MyType{T,N}")), ZT in (:T, Symbol("MyType{T,N}"))
    XT == YT == ZT == :T && continue
    ex = """
        function Base.muladd(x::$XT,y::$YT,z::$ZT) where {T,N}
            res = muladd(f(x),f(y),f(z))
            MyType{eltype(res),N}(res)
        end
    """
    println(ex)
    eval(Meta.parse(ex))
end

The reason I'm parsing a string and not building the expression directly is that I couldn't figure out how to interpolate Symbol("MyType{T,N}") into the expression, it just produced z::var"MyType{T,N}" instead of z::MyType{T,N}.

The output of the program (notice the print statement printing the expression) is

    function Base.muladd(x::T,y::T,z::MyType{T,N}) where {T,N}
        res = muladd(f(x),f(y),f(z))
        MyType{eltype(res),N}(res)
    end

    function Base.muladd(x::T,y::MyType{T,N},z::T) where {T,N}
        res = muladd(f(x),f(y),f(z))
        MyType{eltype(res),N}(res)
    end

ERROR: StackOverflowError:
Stacktrace:
 [1] top-level scope at none:1
 [2] eval at ./boot.jl:331 [inlined]
 [3] eval(::Expr) at ./client.jl:467
 [4] top-level scope at ./none:10

[yuyichao]

The error is unrelated to eval.

julia> struct MyType{T,N}
           x::T
       end

julia> f(x) = x
f (generic function with 1 method)

julia> f(x::MyType) = x.x
f (generic function with 2 methods)

julia> function Base.muladd(x::T,y::T,z::MyType{T,N}) where {T,N}
           res = muladd(f(x),f(y),f(z))
           MyType{eltype(res),N}(res)
       end

julia> function Base.muladd(x::T,y::MyType{T,N},z::T) where {T,N}
           res = muladd(f(x),f(y),f(z))
           MyType{eltype(res),N}(res)
       end
ERROR: StackOverflowError:
Stacktrace:
 [1] top-level scope
   @ REPL[5]:1

---

And for your question, Symbol("MyType{T,N}") is var"MyType{T,N}" and that's why you are not getting anything else. Use quote instead, i.e. :(MyType{T,N}), which will in general just give you whatever corresponds to the thing quoted.

[baggepinnen]

Thanks for the clarification on Expr/Symbol!

[martinholters]

julia> typeintersect(Tuple{T, Ref{T}, T} where {T}, Tuple{T, T, Ref{T}} where {T})
ERROR: StackOverflowError:

Interestingly, swapping the arguments:

julia> typeintersect(Tuple{T, T, Ref{T}} where {T}, Tuple{T, Ref{T}, T} where {T})
Tuple{Union{T1, T2},Union{Ref{T1}, T2},Union{Ref{T2}, T1}} where T2<:Ref{T} where T>:(Union{T, Ref{T}} where T) where T1<:Ref{T} where T>:(Union{T, Ref{T}} where T)

Where I can't wrap my head around whether that makes any sense.