Combinator for alternative in case of failure

[Michael Peyton Jones]

I'd like a function takes two generators, a and b, and yields the value of a if a succeeds, and otherwise tries b. This would be useful for
running a generator which you expect to often fail, but for which you have an alternative that is guaranteed to succeed.

I thought this would be the behaviour of Gen.option, but apparently not - this delegates to Gen.oneOf, which picks randomly between
Some(value) and None.

It's not entirely obvious to me how to write such a function - any ideas?

Michael

[Matthew Pocock]

Hi Michael,

I'm guessing that you can use .getOrElse on the first result, so a.getOrElse(b), something like that. I can't see it now, but there is a variant were b is the raw type, and another variant were b is also an option.

Matthew

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Dr Matthew Pocock

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[Michael Peyton Jones]

Doh, yes, that works. It's a little annoying, as you need to write it from inside the `org.scalacheck` package so you actually have access to the functions to create a `Gen`, but worse things happen at sea.

def orElse[T](g1: Gen[T], g2: Gen[T]): Gen[T] = Gen.gen { p => Gen.r(g1(p) orElse g2(p)) }

M

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[Michael Peyton Jones]

... of course I immediately realise that you don't actually need the internal functions at all. Doh x2

def orElse[T](g1: Gen[T], g2: Gen[T]): Gen[T] = Gen.parameterized { p => Gen.fromOption(g1(p) orElse g2(p)) }

M