introduce a Pair API (alternative to make)

README.md

@@ -41,6 +41,16 @@ is omitted). For example, `rand(make(Array, 2, 3), 3)` creates an array of matri
 Of course, `make` is not necessary, in that the same can be achieved with an ad hoc `struct`,
 which in some cases is clearer (e.g. `Normal(m, s)` rather than something like `make(Float64, Val(:Normal), m, s)`).
 
+As an experimental feature, the following alternative API is available:
+- `rand(T => x)` is equivalent to `rand(make(T, x))`
+- `rand(T => (x, y, ...))` is equivalent to `rand(make(T, x, y, ...))`
+
+This is for convenience only (it may be more readable), but may be less efficient due to the
+fact that the type of a pair containing a type doesn't know this exact type (e.g. `Pair => Int`
+has type `Pair{UnionAll,DataType}`), so `rand` can't infer the type of the generated value.
+Thanks to inlining, the inferred types can however be sufficiently tight in some cases
+(e.g. `rand(Complex => Int, 3)` is of type `Vector{Complex{Int64}}` instead of `Vector{Any}`).
+
 Point 3) allows something like `rand(1:30, Set, 10)` to produce a `Set` of length `10` with values
 from `1:30`. The idea is that `rand([rng], [S], Cont, etc...)` should always be equivalent to
 `rand([rng], make(Cont, [S], etc...))`. This design goes somewhat against the trend in `Base` to create
@@ -204,6 +214,12 @@ julia> collect(Iterators.take(Uniform(1:10), 3)) # distributions can be iterated
   7
  10
   5
+
+julia> rand(Complex => Int) # equivalent to rand(make(Complex, Int)) (experimental)
+4610038282330316390 + 4899086469899572461im
+
+julia> rand(Pair => (String, Int8)) # equivalent to rand(make(Pair, String, Int8)) (experimental)
+"ODNXIePK" => 4
 ```
 
 In some cases, the `Rand` iterator can provide efficiency gains compared to

src/sampling.jl

@@ -154,12 +154,14 @@ Sampler(RNG::Type{<:AbstractRNG}, c::Categorical, n::Repetition) =
 
 ## random elements from pairs
 
+#= disabled in favor of a special meaning for pairs
+
 Sampler(RNG::Type{<:AbstractRNG}, t::Pair, n::Repetition) =
     SamplerSimple(t, Sampler(RNG, Bool, n))
 
 rand(rng::AbstractRNG, sp::SamplerSimple{<:Pair}) =
     @inbounds return sp[][1 + rand(rng, sp.data)]
-
+=#
 
 ## composite types
 
@@ -559,3 +561,22 @@ let b = UInt8['0':'9';'A':'Z';'a':'z'],
 
     rand(rng::AbstractRNG, sp::SamplerTag{Cont{String}}) = String(rand(rng, sp.data.first, sp.data.second))
 end
+
+
+## X => a / X => (a, as...) syntax as an alternative to make(X, a) / make(X, a, as...)
+
+# this is experimental
+
+@inline Sampler(RNG::Type{<:AbstractRNG}, (a, b)::Pair{<:Union{DataType,UnionAll}},
+                r::Repetition) =
+    b isa Tuple ?
+        Sampler(RNG, make(a, b...), r) :
+        Sampler(RNG, make(a, b), r)
+
+# nothing can be inferred when only the pair type is available
+@inline gentype(::Type{<:Pair{<:Union{DataType,UnionAll}}}) = Any
+
+@inline gentype((a, b)::Pair{<:Union{DataType,UnionAll}}) =
+    b isa Tuple ?
+        gentype(make(a, b...)) :
+        gentype(make(a, b))

test/runtests.jl

@@ -565,6 +565,16 @@ end
     @test rand(make(Float64)) isa Float64
 end
 
+@testset "rand(T => x) & rand(T => (x, y, ...))" begin
+    @test rand(Complex => Int) isa Complex{Int}
+    @test rand(Pair => (String, Int8)) isa Pair{String,Int8}
+    @test_throws ArgumentError rand(1=>2) # should not call rand(make(1, 2))
+
+    @test rand(Complex => Int, 3) isa Vector{Complex{Int}}
+    @test rand(Pair => (String, Int8), Set, 3) isa Set{Pair{String,Int8}}
+end
+
+
 ## @rand
 
 struct Die