fix @Half macro (#8)

Author: cscherrer

src/combinators/half.jl

@@ -11,8 +11,10 @@ end
 
 unhalf(μ::Half) = μ.parent
 
-basemeasure(μ::Half) = basemeasure(μ.parent)
+basemeasure(μ::Half) = WeightedMeasure(logtwo, basemeasure(unhalf(μ)))
 
 function Base.rand(rng::AbstractRNG, T::Type, μ::Half)
     return abs(rand(rng, T, unhalf(μ)))
 end
+
+logdensity(μ::Half, x) = (x < 0) * (-Inf) + (x > 0) * logdensity(unhalf(μ), x)

src/macros.jl

@@ -168,7 +168,7 @@ function _half(__module__, ex)
     @match ex begin
         :($dist) => begin
             halfdist = esc(Symbol(:Half, dist))
-            
+            dist = esc(dist)
             quote
                 $halfdist(args...) = Half($dist(args...))
                 $halfdist(;kwargs...) = Half($dist(;kwargs...))

test/runtests.jl

@@ -151,6 +151,13 @@ end
     end
 end
 
+@testset "Half" begin
+    Normal() = ∫exp(x -> -0.5x^2, Lebesgue(ℝ))
+    @half Normal
+    @test logdensity(HalfNormal(), -0.2) == -Inf
+    @test logdensity(HalfNormal(), 0.2) == logdensity(Normal(), 0.2)
+end
+
 # import MeasureBase.:⋅
 # function ⋅(μ::Normal, kernel) 
 #     m = kernel(μ)