Brandon S. Allbery KF8NH wrote:
You can define a set of valid transformations, have the interpreter
log each transformation, and verify that all are correct (that is,
that both the transformation and the logged result are correct.
This assumes the interpreter can be resolved down to a sufficiently
simple set of transformations; if not, you're right back at having the
tester being the interpreter itself. Note that you don't check if the
transformation plan for the program matches a specified list, just
that all transformations are correct. (Just remember that "logic is
an organized way of going wrong with confidence.")
The amusing (?) part is that the interpretter itself is essentially
quite simple. I've implemented it several times before now. But what I'm
trying to do it make it print out elaborately formatted execution traces
so that a human user can observe how execution proceeds. This transforms
an essentially simple algorithm into something quite nausiatingly
complex, with many subtle bugs and issues.
Still, I guess it's not news to anybody that proof-of-concept programs
are much easier that real-world implementations.
One thing I could do is have QuickCheck build arbitrary expressions, run
those through the pretty printer, and then run the result back through
the parser and check that I get the same expression. Can anybody tell me
how to get QuickCheck to build arbitrary expressions though? Let's say I had
data Expression = Var String | Apply Expression Expression | Lambda
String Expression
How would you go about building random expression trees?
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