[Haskell-cafe] QuickCheck, (Ord a)=> [a] -> Property problem

[Daniel Fischer]

On Wednesday 20 April 2011 11:43:08, larry.liuxinyu wrote:
> Hi,
> 
> I found there is similar question as:
> http://groups.google.com/group/haskell-cafe/browse_thread/thread/7439262
> e9ac80dd2/91ca18e11ff00649?lnk=gst&q=QuickCheck+Ord+a#91ca18e11ff00649
> 
> I am still think it's very strange. For example:
> 
> prop_foo :: (Ord a) => [a] -> Property
> prop_foo xs = not (null xs) ==> maximum xs == minimum xs
> 
> This is an extreme case that the property is always wrong.

Not always, replicate n x has this property.

> 
> However, QuickCheck produces:
> *Main> test prop_foo
> OK, passed 100 tests.

> 
> This works at least, However, since 'a'<'b', they are order-able, what
> if I want to test prop_foo works for char?

ghci> test (prop_foo :: [Char] -> Property)

You have to determine the type, by an explicit signature or by context.