PropCheck.shrink

You're seeing just the macro shrink, go back to PropCheck module for more information.
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shrink(generator, alt_gens)

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Defines the shrinking of a generator.

shrink creates a type whose instances are generated by evaluating the statement block generator (this may evaluate to a type, which will then be generated recursively). If an instance of such a type is to be shrunk, the generators in alt_gens are first run to produce hopefully simpler instances of the type. Thus, the generators in the second argument should be simpler than the default. The simplest ones should be at the front of the list, since those are the generators preferred by the shrinking subsystem. Like the main generator, the alternatives may also evaluate to a type, which is generated recursively.

    iex> use PropCheck
    iex> quickcheck(
    ...>   forall n <- shrink(pos_integer(), [0]) do
    ...>     rem(n, 2) == 0
    ...>   end)
    false