[pron]

> zig does not seem to have something that looks like concepts ... nor a way to put restrictions on a given type passed to a function or series of function.

Of course it does. Just place a comptime assertion on the @TypeOf a var.

> would require in zig to have a comptime test for the "foobar" nature of T in every function, which looks like a downgrade and does not expose the requirements cleanly in the function interface.

Cleanliness is a matter of personal aesthetic preference, and a rich type specification "in every function" is not less work or less clear than a comptime assertion in every function. In either case you get a clear compilation error. Requirements are never fully specified in the function's interface unless you have dependent types or contracts (both with their own problems and tradeoffs). Even Haskell has partial subroutines. What portions of the spec that is already enforced at compile type should be exposed in the signature -- keeping in mind that it will never be all of them -- is a matter of taste. Zig might shift things around a bit, but the capability to enforce a specification that is at least as strong as structural types at compile-time is already there.