Generating a random rule for property based test - erlang

Question

I am using Triq (erlang quickcheck) and I am having trouble generating a set of nice rules for my program.
What I want to generate are things that looks like this:
A -> B
where I would like to provide A and the size of B, with latter not having any dupicates.
For example, if I say generate me rules with L.H.S. of [a] and R.H.S. of size 4 (ie. A = [a] and size(B) = 4) I would like to get something like this:
{rule, [a], [1,2,4,5]}
{rule, [a], [a,d,c,e]}
{rule, [a], [q,d,3,4]}
Note, I don't want any dupicates in B (this is the part I'm having trouble with). Also, it doesn't really matter what B is made up of - it can be anything as long as it is distinct and without dupicates.
My spec is far too messy to show here, so I'd rather not.

Answer 1

I am not familiar with Triq, but in PropEr and Quviq's Qickcheck you can use ?SUCHTHAT conditions that filter 'bad' instances.
If a generated instance does not satisfy a ?SUCHTHAT constraint it is discarded and not counted as a valid test. You could use this mechanism to generate lists of the specified size (i.e. what PropEr calls 'vectors') and then discard those that have duplicates, but I think that too many instances would then be discarded (see also the link).
It is usually more efficient to tinker with the generator so that all instances are valid, in your case by e.g. generating (3) X-times as many elements, removing duplicates and keeping as many as you need. This can still fail, and it will fail, so you need to guard against it.
Here is a generator for your case, in PropEr, together with a dummy property:
-module(dummy).
-export([rule_prop/0]).
-include_lib("proper/include/proper.hrl").
-define(X, 5).
rule_prop() ->
?FORALL(_, rule_gen(integer(), 4, integer()), true).
rule_gen(A, SizeB, TypeB) ->
?LET(
EnoughB,
?SUCHTHAT(
NoDupB,
?LET(
ManyB,
vector(?X * SizeB, TypeB),
no_dups(ManyB)
),
length(NoDupB) >= SizeB
),
begin
B = lists:sublist(EnoughB, SizeB),
{rule, A, B}
end).
no_dups([]) ->
[];
no_dups([A|B]) ->
[A | no_dups([X || X <- B, X =/= A])].