[Python-ideas] explicitation lines in python ? (original) (raw)

Carl M. Johnson cmjohnson.mailinglist at gmail.com
Mon Jul 12 08:50:28 CEST 2010

One more quick thought about the advantage of a where-clause. Often times, there is thought of creating the equivalent of Haskell-style pattern matching using decorators. For example PJE has worked on creating "generic functions." One problem with using a decorator for this is that for use cases more complicated than just matching on type, the matcher itself needs to be a function that looks at the arguments then returns true or false based on whether they match a pattern. So, a proper decorator would need to take two functions, one to do the matching and one to actually be the body of the function. You can do this to some extent with lambdas or decorating and redecorating, but it quickly becomes a little tedious. With a where-clause one might instead write:

fib = base(cond, action) where: def cond(n): return n in (0, 1) def action(n): return 1

fib.add_match(cond, action) where: def cond(n): return isinstance(n, int) and n > 1 def action(n): return n + fib(n - 1)

And also for the property decorator. GvR made up a nice way of redecorating with properties, so this is a moot point now, but if we had had a where-clause before that, we could have instead written:

myprop = property(getter, setter, deleter) where: def getter(self): etc. etc.

OK, that’s as much advocacy as I feel like doing. See you all again in 6 months, when something like this is proposed again. ;-)

-- Carl

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