Promotion with rationals when constructing arrays

[ranocha]

This was reported originally by @ketch in ranocha/BSeries.jl#96

Using the recommendation in https://discourse.julialang.org/t/symbolic-sqrt-2-and-sin-pi/58633 to represent a square root symbolically does not work well when constructing an array of symbolic expressions and rational numbers:

julia> using Pkg; Pkg.activate(temp = true); Pkg.add("Symbolics")
  Activating new project at `/tmp/jl_r7ULJP`
    Updating registry at `~/.julia/registries/General.toml`
   Resolving package versions...
    Updating `/tmp/jl_r7ULJP/Project.toml`
  [0c5d862f] + Symbolics v5.1.0
...

julia> sqrt3 = Symbolics.Term(sqrt, [3])
sqrt(3)

julia> [1, sqrt3]
2-element Vector{Any}:
 1
  sqrt(3)

Note the Vector{Any}.

[bowenszhu]

Symbolics.jl has corresponding promotion rules.

Symbolics.jl/src/num.jl

Lines 98 to 99 in 426eb80

	Base.promote_rule(::Type{<:Number}, ::Type{<:Num}) = Num
	Base.promote_rule(::Type{<:Symbolic{<:Number}}, ::Type{<:Num}) = Num

Normally users are supposed not to call internal constructors. But here symbolic version of sqrt(3) is special and we should wrap it with a Num struct.

julia> using Symbolics

julia> sqrt3 = Symbolics.wrap(Symbolics.Term(sqrt, [3]))           # Note this line
sqrt(3)

julia> [1, sqrt3]
2-element Vector{Num}:
       1
 sqrt(3)

[ranocha]

Thanks! It would be great if this could make it to the docs.