[Haskell-beginners] QuickCheck 2 entering infinite loop

[Dominic Mulligan]

Dear all,

I'm trying to work out how to use QuickCheck 2 (installed via apt-get 
from the Ubuntu repositories) and in the process keep managing to get 
the quickCheck function (called under GHCi 6.10.4) to enter what appears 
to be an infinite loop.  Evidently, I'm doing something wrong ...

Here's the problem: I have a module for types, and it exports a datatype 
called "Type".  I make this an instance of Arbitrary, like so:

instance (Eq a, Arbitrary a) => Arbitrary (Type a) where
    arbitrary = oneof [
      liftM tyVar arbitrary,
      liftM2 tyCon arbitrary arbitrary]

Here, tyVar and tyCon are two functions injecting into Types.

I then write two silly properties to test whether this works.  The first 
property:

  prop_InjectTypeVarFreeVar var =
    (freeTypeVars . tyVar) var == [tyVar var]

Works fine.  A call to quickCheck gives me the familiar "100 tests 
passed" message.  The second property:

  prop_InjectTypeConFreeVars args =
    (freeTypeVars . (tyCon "Fun")) args == (concatMap freeTypeVars args)

Sends quickCheck into an infinite loop of some sort.  It manages to get 
through 5-6 tests before GHCi is brought to its knees.  Before I go 
crazy, can anybody explain what I'm doing wrong?

Many thanks in advance,
Dominic