Clarify usage of `sum` inside macros

[joaquimg]

The sum inside macros does not need the init for empty collections:

julia> @expression(m, sum(2 for i in 1:0))
0.0

julia> sum(2 for i in 1:0)
ERROR: ArgumentError: reducing over an empty collection is not allowed; consider supplying `init` to the reducer

However, AI insists in:

julia> @expression(m, sum(2 for i in 1:0), init = AffExp(0.0))
0.0

We should clarify in macros docs / getting started / performance / good practices.

[odow]

@expression(m, sum(2 for i in 1:0), init = AffExp(0.0))

Woah woah woah. What is this? Did you mean to make the init inside the sum?

[odow]

I guess it was just a typo:

julia> using JuMP

julia> model = Model();

julia> @expression(model, sum(2 for i in 1:0))
0.0

julia> @expression(model, sum(2 for i in 1:0; init = zero(AffExpr)))
0

julia> @expression(model, sum(2 for i in 1:0)) |> typeof
Float64

julia> @expression(model, sum(2 for i in 1:0; init = zero(AffExpr))) |> typeof
AffExpr (alias for GenericAffExpr{Float64, GenericVariableRef{Float64}})

julia> @expression(model, sum(2 for i in 1:0), init = zero(AffExpr))
ERROR: LoadError: At REPL[7]:1: `@expression(model, sum((2 for i = 1:0)), init = zero(AffExpr))`: unsupported keyword argument `init`.
Stacktrace:
 [1] error(::String, ::String)
   @ Base ./error.jl:54
 [2] (::JuMP.Containers.var"#error_fn#build_error_fn##0"{LineNumberNode, String})(str::String)
   @ JuMP.Containers ~/git/jump-dev/JuMP/src/Containers/macro.jl:345
 [3] parse_macro_arguments(error_fn::JuMP.Containers.var"#error_fn#build_error_fn##0"{…}, args::Tuple{…}; valid_kwargs::Vector{…}, num_positional_args::UnitRange{…})
   @ JuMP.Containers ~/git/jump-dev/JuMP/src/Containers/macro.jl:56
 [4] var"@expression"(__source__::LineNumberNode, __module__::Module, input_args::Vararg{Any})
   @ JuMP ~/git/jump-dev/JuMP/src/macros/@expression.jl:68
in expression starting at REPL[7]:1
Some type information was truncated. Use `show(err)` to see complete types.

So to clarify: the ask is for documentation that you don't need to use init inside the macros, but it's okay if you do?

[joaquimg]

My bad, typo.

I would even recommend not to use init.

Or do we want that to ensure type stability?

I might need to check docs...

[odow]

The other issue with init is that it requires knowing what the type of the expression is. There are plenty of cases where the result is a QuadExpr, or a Vector of something.