[Haskell-cafe] QuickCheck Arbitrary - infinite recursion on recursive GADTs?

2007-06-04 Thread Marc Weber 
example:

data A = A INt
   | B [A]

instace Arbitrary A where
  arbitrary = oneof [ liftM A arbitrary
, liftM B arbitrary
]

But now QuickCheck will propably create a test value
A ( B [ A ( B [ A  - no end

Is there an easy QuickCheck way to prevent this?

I can think of:
a) using sized to pass when to stop (decrement the value ..)
b) using some type hackary
  data Recursive a = R a
  then use arbitrary :: R ( R ( a ))
   and remove one R each time..
c) pass a recursion indicator manually and decrement that...
  
Which would you choose?

Marc
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Re: [Haskell-cafe] QuickCheck Arbitrary - infinite recursion on recursive GADTs?

2007-06-04 Thread Bryan O'Sullivan 

Marc Weber wrote:


data A = A INt
   | B [A]

instace Arbitrary A where
  arbitrary = oneof [ liftM A arbitrary
, liftM B arbitrary
]

But now QuickCheck will propably create a test value
A ( B [ A ( B [ A  - no end

Is there an easy QuickCheck way to prevent this?


There are two successive sections in the QuickCheck manual that cover 
exactly this topic.


http://www.cs.chalmers.se/~rjmh/QuickCheck/manual_body.html#15

b
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