Undocumented or unintended change in interface for defining distributions

[s-broda]

The following code works on 0.16.4

using Distributions
using Random
import Random: rand
using StatsFuns.RFunctions: tdistrand
export StdT
"""
    StdT <: Distribution{Univariate, Continuous}
Standardized t distribution.
"""
struct StdT <: Distribution{Univariate, Continuous}
    v::Float64
end
rand(d::StdT)  =  tdistrand(d.v)*sqrt((d.v-2)/d.v)
Random.seed!(1)
@show rand(StdT(4), 1)
end

but errors on 0.17.0 with

ERROR: LoadError: MethodError: no method matching iterate(::Main.Repro.StdT)
Closest candidates are:
  iterate(::Core.SimpleVector) at essentials.jl:568
  iterate(::Core.SimpleVector, ::Any) at essentials.jl:568
  iterate(::ExponentialBackOff) at error.jl:199
  ...
Stacktrace:
 [1] copyto!(::Array{Float64,1}, ::Main.Repro.StdT) at .\abstractarray.jl:668
 [2] _collect(::UnitRange{Int64}, ::Main.Repro.StdT, ::Base.HasEltype, ::Base.HasLength) at .\array.jl:550
 [3] collect(::Main.Repro.StdT) at .\array.jl:544
 [4] #quantile#51(::Bool, ::Function, ::Main.Repro.StdT, ::Float64) at C:\cygwin\home\Administrator\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.1\Statistics\src\Statistics.jl:916
 [5] quantile(::Main.Repro.StdT, ::Float64) at C:\cygwin\home\Administrator\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.1\Statistics\src\Statistics.jl:916
 [6] rand(::Random.MersenneTwister, ::Main.Repro.StdT) at C:\Users\broda\.julia\packages\Distributions\fMt8c\src\univariates.jl:177
 [7] rand! at C:\Users\broda\.julia\packages\Distributions\fMt8c\src\univariates.jl:167 [inlined]
 [8] rand at C:\Users\broda\.julia\packages\Distributions\fMt8c\src\univariates.jl:160 [inlined]
 [9] rand(::Main.Repro.StdT, ::Int64) at C:\Users\broda\.julia\packages\Distributions\fMt8c\src\genericrand.jl:24
 [10] top-level scope at show.jl:555
in expression starting at C:\Users\broda\Documents\julia\local_packages\Repro\src\Repro.jl:17

Conversely, changing

rand(d::StdT)  =  tdistrand(d.v)*sqrt((d.v-2)/d.v)

to

rand(RNG::Random.AbstractRNG, d::StdT)  =  tdistrand(d.v)*sqrt((d.v-2)/d.v)

works on v0.17.0, but fails on v0.16.4, with the same error. The code works on both versions if both definitions are present.

I can't seem to find anything in the docs about this change.
Thanks!
Simon

[matbesancon]

ok yes this is an issue indeed, sorry about that breaking change. We forgot to release in a long time, which is far from ideal

[matbesancon]

@s-broda digging into what happened, the problem here is that

rand(::StdT, ::Int)

Will boil down to:

rand(::RNG, ::StdT, n)

which will not re-dispatch to rand(d::StdT)

So the problem is that we have a change of required interface to implement: rand(::AbstractRNG, ::YourDist)

[matbesancon]

Mostly a bug from 0.16 I would say, and the error should be more meaningful. If you want to open a PR, I'd be 100% for it

[matbesancon]

So there could be an error on calling quantile(::Dist, p) by default to force an implementation