Comefrom0x10: day 5

A few days have passed building a brand new language. About a day and a half getting most of the grammar worked out and another day and a half building the guts of the interpreter. But now cf0x10 programs can basically run, albeit with a limited set of operators and without entirely correctly scope resolution.

Of course, I spent much of the time on the interpreter working out the ideal semantics for the comefrom statement, a much neglected field of computer science. The natural question is, “what happens when more than one statement comes from a single other statement?”

In Intercal, this is simply an error. Though that might seem like a cowardly evasion, it can simply be credited to Intercal’s being a lower-level language: the Intercal programmer needs to handle these situations explicitly by abstaining and so forth.

Comefrom0x10, on the other hand, is a high-level language designed to minimize runtime errors, so Comefrom0x10 has well-defined semantics for all variations of coming from a place.

First, eligible comefroms receive control in the order they appear in the source. Alone, however, this rule causes trouble breaking out of blocks. Consider this program that prints the Fibonacci sequence up to ten thousand:

fib
  b = 1
  comefrom fib
  a
  next_b = a + b
  a = b
  b = next_b

  comefrom fib if a > 10000

If the first comefrom took precedence, the loop would never end. Changing the first line to “comefrom fib if a < 10000” would fix the problem, but require reconfiguring how the variables a and b get swapped. I think the existing version is more clear: last-first wins allows a common idiom of placing your exit conditions at the end of a block.

The first exception to source order, therefore, is that when multiple comefroms are eligible within the same block, the last one wins.

There are other exceptions, but that’s another article.

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