Composing generators

@composed

While Supposition.jl provides basic generators for a number of objects from Base, quite a lot of Julia code relies on the use of custom structs. At the innermost level, all Julia structs are composed of one or more of these basic types, like Int, String, Vector etc. Of course, we want to be able to generate & correctly shrink these custom structs as well, so how can this be done? Enter @composed, which can do exactly that. Here's how it's used:

using Supposition

const intgen = Data.Integers{Int}()

makeeven(x) = (x÷0x2)*0x2

even_complex = @composed function complex_even(a=intgen, b=intgen)
    a = makeeven(a)
    b = makeeven(b)
    a + b*im
end
example(even_complex, 5)

5-element Vector{Complex{Int64}}:
  2039250369958159498 - 5351598215399239502im
 -3577491071005366610 - 4476213672568630062im
  8190652638782828060 + 8341451288263644814im
 -3650148928031328650 + 5066447988930209878im
 -2437974777491117090 + 7705698290406928824im

In essence, @composed takes a function that is given some generators, and ultimately returns a generator that runs the function on those given generators. As a full-fledged Possibility, you can of course do everything you'd expect to do with other Possibility objects from Supposition.jl, including using them as input to other @composed! This makes them a powerful tool for composing custom generators.

@check function all_complex_even(c=even_complex)
    iseven(real(c)) && iseven(imag(c))
end

In addition, @composed defines the function given to it as well as a regular function, which means that you can call & reuse it however you like:

complex_even(1.0,2.0)

0.0 + 2.0im

Filtering, mapping, and other combinators

filter

Of course, manually marking, mapping or filtering inside of @composed is sometimes a bit too much. For these cases, all Possibility support filter and map, returning a new Data.Satisfying or Data.Map Possibility respectively:

using Supposition

intgen = Data.Integers{UInt8}()

f = filter(iseven, intgen)

example(f, 10)

10-element Vector{UInt8}:
 0x58
 0xca
 0x40
 0x80
 0xd6
 0x26
 0xcc
 0x2c
 0x32
 0x0e

Note that filtering is, in almost all cases, strictly worse than constructing the desired objects directly. For example, if the filtering predicate rejects too many examples from the input space, it can easily happen that no suitable examples can be found:

g = filter(>(typemax(UInt8)), intgen)
example(g, 10)

ERROR: Tried sampling 100000 times, without getting a result. Perhaps you're filtering out too many examples?

It is best to only filter when you're certain that the part of the state space you're filtering out is not substantial.

map

In order to make it easier to directly construct conforming instances, you can use map, transforming the output of one Possibility into a different object:

using Supposition

intgen = Data.Integers{UInt8}()
makeeven(x) = (x÷0x2)*0x2
m = map(makeeven, intgen)

example(m, 10)

10-element Vector{UInt8}:
 0xc4
 0x10
 0xde
 0x14
 0xf0
 0x22
 0x4a
 0x5a
 0xbe
 0x02