Explicit type annotation of MetaGraphs

If I have a function that returns a MetaGraph is it possible to annotate the type?

e.g.

const CustomGraph = MetaGraph(Int64, SimpleDiGraph(Int64), ...)

function create_graph()::CustomGraph
    out = CustomGraph(SimpleDiGraph())
    return out
end

This schema doesn't work, and it says "no method matching ... exists"

Output type annotations are seldom used in Julia, can I ask why you want to do this in the first place?

Also, could you provide a complete runnable example? Here I'm not sure whether CustomGraph is meant to be a type or an object?

Output type annotations are seldom used in Julia, can I ask why you want to do this in the first place?

I want to pass values with the CustomGraph type into other functions. I'm only using the output type as an example. CustomGraph is meant to be a type.

Can you give a complete code example? Usually you would rather constrain the input types of other functions for dispatch purposes.

Can you give a complete code example? Usually you would rather constrain the input types of other functions for dispatch purposes.

e.g.

using Graphs, MetaGraphsNext

const ColorGraph = MetaGraph

function generate_colors(n::Int)::ColorGraph
    graph = Graph(Edge.([(1, 2), (1, 3), (2, 3)]))
    vertices_description = [:red => (255, 0, 0), :green => (0, 255, 0), :blue => (0, 0, 255)]
    edges_description = [
        (:red, :green) => :yellow, (:red, :blue) => :magenta, (:green, :blue) => :cyan
    ]

    return MetaGraph(graph, vertices_description, edges_description, "additive colors")
end

function consume_colors(colors::ColorGraph)
    @info "$(nv(colors))"
end

g = generate_colors(123)
@info "colors: $(typeof(g))"
consume_colors(g)

Instead of having ColorGraph = MetaGraph, would it be possible to have ColorGraph = MetaGraph(Int64, ...), and thus ensure it is not possible to pass in an unrelated MetaGraph into consume_colors?

First of all, argument annotation and output annotation have different behaviors:

argument annotations prevent the method from accepting anything else
output annnotations will try to convert the result if it doesn't have the right type

My take is that output annotations are rarely necessary when you constrain the arguments on downstream methods anyway. With that in mind, you could use the full type of g as a type annotation, but it is rather convoluted and there is one type parameter (the var... at the end) which is a MetaGraphsNext internal.

julia> typeof(g)
MetaGraph{
    Int64,  # Code
    SimpleGraph{Int64},  # Graph
    Symbol,  # Label
    Tuple{Int64, Int64, Int64},  # VertexData
    Symbol,  # EdgeData
    String,  # GraphData
    MetaGraphsNext.var"#MetaGraph##14#MetaGraph##15",  # WeightFunction
    Float64  # Weight
}

So if I had this problem, I would probably define

const ColorGraph = MetaGraph{
    Int64,  # Code
    SimpleGraph{Int64},  # Graph
    Symbol,  # Label
    Tuple{Int64, Int64, Int64},  # VertexData
    Symbol,  # EdgeData
    String,  # GraphData
}

and skip the internal weights type parameter, especially if you don't use it.

First of all, argument annotation and output annotation have different behaviors:
* argument annotations prevent the method from accepting anything else

* output annnotations will try to convert the result if it doesn't have the right type
My take is that output annotations are rarely necessary when you constrain the arguments on downstream methods anyway. With that in mind, you could use the full type of g as a type annotation, but it is rather convoluted and there is one type parameter (the var... at the end) which is a MetaGraphsNext internal.

julia> typeof(g)
MetaGraph{
Int64, # Code
SimpleGraph{Int64}, # Graph
Symbol, # Label
Tuple{Int64, Int64, Int64}, # VertexData
Symbol, # EdgeData
String, # GraphData
MetaGraphsNext.var"#MetaGraph##14#MetaGraph##15", # WeightFunction
Float64 # Weight
}

So if I had this problem, I would probably define

const ColorGraph = MetaGraph{
Int64, # Code
SimpleGraph{Int64}, # Graph
Symbol, # Label
Tuple{Int64, Int64, Int64}, # VertexData
Symbol, # EdgeData
String, # GraphData
}

and skip the internal weights type parameter, especially if you don't use it.

This solution is perfect! Thank you.

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Explicit type annotation of `MetaGraph`s #95

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Explicit type annotation of MetaGraphs #95

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gdalle commented on Oct 6, 2025

gdalle commented on Oct 8, 2025

lenianiva commented on Oct 9, 2025

gdalle commented on Oct 9, 2025

lenianiva commented on Nov 23, 2025

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lenianiva commented on Nov 26, 2025

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Explicit type annotation of `MetaGraph`s #95