> One of the problems with most PL type systems is that they predate ubiquitous networking; so they're not designed to address the problem of describing external data, eg with MIME types, XML DTDs, protobuf schemas or whatever.

The problem existed before "ubiquitous networking" if you realise that networking is just a specific kind of I/O. But I/O from a PL perspective is nothing more than sending or receiving bytes. PLs usually do not impose a format and a meaning on I/O because it would restrict the capabilities of the applications and libraries.

One could argue that PLs should impose such restrictions just like some PLs impose restrictions on memory access ("managed languages"). However, a huge difference between memory and I/O - and especially networking - is that they are unreliable by default: transmission errors, disconnections, lags, etc.

But even if you solve that, your PL can only speak to itself, which quite significantly restricts its practical usefulness. As far as I know, only languages specialized in distributed computing do that.

But for a general purpose language, high level networking is usually not something you want to be carved in stone (in the language's specs). You'd rather want to leave that in libraries, that can be replaced and enhanced.