Editing Introduction to QuickCheck1 (section)

== Testing take5 ==

The first step to testing with QuickCheck is to work out some properties
that are true of the function, for all inputs. That is, we need to find
''invariants''.

A simple invariant might be:
    <math>\forall~s~.~length~(take5~s)~=~5</math>

So let's write that as a QuickCheck property:
<haskell>
\s -> length (take5 s) == 5
</haskell>

Which we can then run in QuickCheck as:
<haskell>
*A> quickCheck (\s -> length (take5 s) == 5)
Falsifiable, after 0 tests:
""
</haskell>

Ah! QuickCheck caught us out. If the input string contains less than 5
filterable characters, the resulting string will be less than 5
characters long. So let's weaken the property a bit:
    <math>\forall~s~.~length~(take5~s)~\le~5</math>

That is, take5 returns a string of at most 5 characters long. Let's test
this:    
<haskell>
*A> quickCheck (\s -> length (take5 s) <= 5)
OK, passed 100 tests.
</haskell>

Good!