I would like a feature like this as well… Like an assert. @type_stable_assert 
1 Like
The hard part is finding a set of static rules for a subset of Julia, but it does seem like it might be possible to find such a rule set and let people declare sections of code to follow those rules and accordingly get guaranteed tight type bounds and compile-time type checking. Definitely an interesting area for future work.
5 Likes
On the other hand, you can type assert return types, fun(x)::Int, which automatically makes the function type stable. This is the way C guarantees type stability.
If you mean that this
function fun(x)::Int
...
end
asserts the return-type, then you’re wrong (otherwise ignore me!). It converts the type:
julia> function f(x)::Int
x
end
f (generic function with 1 method)
julia> f(Int32(3))
3
julia> Int32(3)::Int
ERROR: TypeError: typeassert: expected Int64, got Int32
My point is that it guarantees type-stability. Wherever you call fun you are sure the return type is an Int. Also, it only converts the type if it is “convertable”. fun("hello") throws an error.