Erlang Central

Recursive Genereators

Revision as of 11:07, 20 October 2008 by TribbleFaith467 (Talk | contribs)

(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)


Thomas Arts

Recursive Generators

Assume you write a function that takes a recursively defined data type as input and you like to test it with QuickCheck. How would you do that?

Example - Cartesian product of sets

We add the function 'product/2' to Erlang's sets module (in standard library) computing the Cartesian product of two sets. A new set can be constructed by associating every element of one set with every element of another set. The Cartesian product of two sets A and B, denoted by A × B is the set of all ordered pairs (a, b) such that a is a member of A and b is a member of B.

For example,

> S1 = sets:from_list([1,2]),
  S2 = sets:from_list([red,white,blue]),

In the fashion of test driven development, we start by writing down a property that should hold.

prop_relative_complement() ->
                     [ {I,C} || I<-sets:to_list(S1), C<-sets:to_list(S2)])

%% Since lists originating from sets may have the elements in different
%% orders, we need to compare that they contain the same elements.
equal(L1,L2) ->
    (L1 -- L2) == [] andalso (L2 -- L1) == [].

Surely, we could implement the product function with the list comprehension above, but we want a more efficient implementation. The test, however, need not at all be efficient.