matshell.jl

function cmatshell(m::C.Mat, y::C.Vec, x::C.Vec)
    f_ = MatGetContext(m)
    f = Function(unsafe_object_ptr(f_)) # get actual Julia function
    f(Vec(y), Vec(x))
end

function Mat(f::Function, ...)
    m = MatCreate(...)
    m.data = f
    MatSetContext(f)
    MatSetShellOperation(m, cfunction(cmatshell)
end