是否可以表示依赖于另一种参数类型的字段类型？

Question

作为一个人工示例，假设我有一个参数结构，其中T <: AbstractFloat

mutable struct Summary{T<:AbstractFloat}

    count
    sum::T

end

我想在T === Float16 时将count 字段输入为UInt16，在T === Float32 时输入为UInt32，在所有其他情况下输入为UInt64。

我目前的方法是为 count 字段使用联合类型 Union{UInt16, UInt32, UInt64}

module SummaryStats

export Summary, avg

const CounterType = Union{UInt16, UInt32, UInt64}

mutable struct Summary{T<:AbstractFloat}

    count::CounterType
    sum::T
    # explicitly typed no-arg constructor
    Summary{T}() where {T<:AbstractFloat} = new(_counter(T), zero(T))
end

# untyped no-arg constructor defaults to Float64
Summary() = Summary{Float64}()

function avg(summary::Summary{T})::T where {T <: AbstractFloat}
    if summary.count > zero(_counter(typeof(T)))
        summary.sum / summary.count
    else
        zero(T)
    end
end

# internal helper functions, not exported
Base.@pure _counter(::Type{Float16})::UInt16 = UInt16(0)
Base.@pure _counter(::Type{Float32})::UInt32 = UInt32(0)
Base.@pure _counter(::DataType)::UInt64 = UInt64(0)

end # module

这似乎可行，但显然@code_warntype 对count 字段的联合类型不满意。

我想知道是否有可能根据上面列出的规则以某种方式计算出正确的具体类型？

Answer 1

"outer-only" constructors 主要用于这些用例：

julia> const CounterType = Union{UInt16, UInt32, UInt64}
Union{UInt16, UInt32, UInt64}

julia> mutable struct Summary{T<:AbstractFloat, S<:CounterType}
           count::S
           sum::T
           function Summary{T}() where {T<:AbstractFloat}
               S = T === Float16 ? UInt16 : 
                   T === Float32 ? UInt32 :
                   T === Float64 ? UInt64 : throw(ArgumentError("unexpected type: $(T)!"))
               new{T,S}(zero(S), zero(T))
           end
       end

julia> Summary() = Summary{Float64}()
Summary

julia> function avg(summary::Summary{T})::T where {T <: AbstractFloat}
       if summary.count > zero(summary.count)
           summary.sum / summary.count
       else
           zero(T)
       end
   end
avg (generic function with 1 method)

julia> avg(Summary())
0.0

julia> @code_warntype avg(Summary())
Body::Float64
1 ─ %1 = (Base.getfield)(summary, :count)::UInt64
│   %2 = (Base.ult_int)(0x0000000000000000, %1)::Bool
└──      goto #3 if not %2
2 ─ %4 = (Base.getfield)(summary, :sum)::Float64
│   %5 = (Base.getfield)(summary, :count)::UInt64
│   %6 = (Base.uitofp)(Float64, %5)::Float64
│   %7 = (Base.div_float)(%4, %6)::Float64
└──      return %7
3 ─      return 0.0

julia> @code_warntype avg(Summary{Float32}())
Body::Float32
1 ─ %1 = (Base.getfield)(summary, :count)::UInt32
│   %2 = (Base.ult_int)(0x00000000, %1)::Bool
└──      goto #3 if not %2
2 ─ %4 = (Base.getfield)(summary, :sum)::Float32
│   %5 = (Base.getfield)(summary, :count)::UInt32
│   %6 = (Base.uitofp)(Float32, %5)::Float32
│   %7 = (Base.div_float)(%4, %6)::Float32
└──      return %7
3 ─      return 0.0f0