Saturday, May 2, 2015

Combining procedures in Wombat

The need to combine procedures occurs a few times in the Wombat design. It is something that forced itself on me, and I'm surprised it isn't a common feature of programming languages.

The most obvious example is the case statement. In Wombat the conditions which select the operation are not separated. Instead there is just a list of procedures which contain the tests inside them, and fail harmlessly ( when the condition is not met. So in case it is normally expected that exactly one of the procedures will succeed, and the result of that procedure is the result of the case statement. There is also a firstCase statement that tries the procedures sequentially till one succeeds, but in case the procedures are executed in parallel (conceptually).

The corresponding caseP procedure is curried and with the parameters reversed, so it has type
List(X->Y) -> X -> Y
All the procedures in the list are passed the X parameter and evaluated in parallel. Normally all except one of the procedures fails harmlessly ( It is also permitted for more than one to  complete as long as they all return the same value. Note that for these purposes they might be different types connected by an isA relationship. For example Int isA Rational, so it is ok if one procedure returns 2 and the other returns Rational(2,1).

If the results of the multiple successful procedures are not equal then they can be combined in a way that ensures their compatibility at the later point of use (if their types support that). In particular if more than one procedure is returned then the result of the caseP is a procedure which, when later executed, will evaluate all procedures and:

  • If all but one fail harmlessly then return the result of that one;
  • If more than one returns then they need to agree on the result;
  • Except that it is also permissible to combine the results if procedures are returned.
So, neatly, the way procedures are combined is with the caseP procedure.

We've glossed over a couple of things. During the parallel evaluation of procedures it is permitted for there to be side effects as long as they all have exactly the same side effects in the same order, in which case that side effect happens just once. Also what if the result is a structure that contains some procedures? Then the structure must be the same with the corresponding values the same or, if procedure values, combined.

[see also:]

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