It would be nice to be able to check if a function is pure and fast for free at runtime using effect analysis. is_pure_and_fast(f, T::Type{<:Tuple}) returns true if f, applied to inputs of type T, is pure an fast. False negatives are okay.

is_pure_and_fast(identity, Tuple{Matrix{Foat64}}) == true
is_pure_and_fast(x -> x^3 + x^2 - 3, Tuple{Foat64}) == true
is_pure_and_fast(x -> iterated_md5_hash(x, iters=10_000), Tuple{String}) == false
is_pure_and_fast(x -> rand(), Tuple{}) == false

It would also be nice to get all of effect analysis for free at runtime.

The motivating usecase here is when computing findall, it is often more efficient to traverse the input twice than to incrementally build an output, but only if f is fast and pure.

function findall(f, x::AbstractArray)
    if is_pure_and_fast(f ∘ getindex, Tuple{typeof(x), Int}) # evaluated at compile time
        _findall(Iterators.map(f, x))
    else
       _findall(f.(x))
    end
end

function _findall(x)
    out = Vector{Int}(undef, count(x)+1)
    j = 1
    @inbounds for (i,v) in enumerate(x)
        out[j] = i
        j += v
    end
    resize!(out, j-1)
    out
end