Fix for unwrap_left_right_vns (#297)

I just noticed a bug I introduced in a recent PR when looking at #295 . This PR fixes it. I'll add tests, a sec. @yebai

Project.toml

@@ -1,6 +1,6 @@
 name = "DynamicPPL"
 uuid = "366bfd00-2699-11ea-058f-f148b4cae6d8"
-version = "0.13.0"
+version = "0.13.1"
 
 [deps]
 AbstractMCMC = "80f14c24-f653-4e6a-9b94-39d6b0f70001"

src/compiler.jl

@@ -90,6 +90,35 @@ left-hand side of a `.~` expression such as `x .~ Normal()`.
 
 This is used mainly to unwrap `NamedDist` distributions and adjust the indices of the
 variables.
+
+# Example
+```jldoctest; setup=:(using Distributions)
+julia> _, _, vns = DynamicPPL.unwrap_right_left_vns(MvNormal(1, 1.0), randn(1, 2), @varname(x)); map(string, vns)
+2-element Vector{String}:
+ "x[:,1]"
+ "x[:,2]"
+
+julia> _, _, vns = DynamicPPL.unwrap_right_left_vns(Normal(), randn(1, 2), @varname(x[:])); map(string, vns)
+1×2 Matrix{String}:
+ "x[:][1,1]"  "x[:][1,2]"
+
+julia> _, _, vns = DynamicPPL.unwrap_right_left_vns(Normal(), randn(3), @varname(x[1])); map(string, vns)
+3-element Vector{String}:
+ "x[1][1]"
+ "x[1][2]"
+ "x[1][3]"
+
+julia> _, _, vns = DynamicPPL.unwrap_right_left_vns(Normal(), randn(1, 2, 3), @varname(x)); map(string, vns)
+1×2×3 Array{String, 3}:
+[:, :, 1] =
+ "x[1,1,1]"  "x[1,2,1]"
+
+[:, :, 2] =
+ "x[1,1,2]"  "x[1,2,2]"
+
+[:, :, 3] =
+ "x[1,1,3]"  "x[1,2,3]"
+```
 """
 unwrap_right_left_vns(right, left, vns) = right, left, vns
 function unwrap_right_left_vns(right::NamedDist, left, vns)
@@ -103,7 +132,7 @@ function unwrap_right_left_vns(
     # for `i = size(left, 2)`. Hence the symbol should be `x[:, i]`,
     # and we therefore add the `Colon()` below.
     vns = map(axes(left, 2)) do i
-        return VarName(vn, (vn.indexing..., Colon(), Tuple(i)))
+        return VarName(vn, (vn.indexing..., (Colon(), i)))
     end
     return unwrap_right_left_vns(right, left, vns)
 end

test/Project.toml

@@ -20,7 +20,7 @@ Zygote = "e88e6eb3-aa80-5325-afca-941959d7151f"
 
 [compat]
 AbstractMCMC = "2.1, 3.0"
-AbstractPPL = "0.1.4, 0.2"
+AbstractPPL = "0.2"
 Bijectors = "0.9.5"
 Distributions = "< 0.25.11"
 DistributionsAD = "0.6.3"