Make Test a weak dependency

Project.toml

@@ -1,6 +1,6 @@
 name = "ChangesOfVariables"
 uuid = "9e997f8a-9a97-42d5-a9f1-ce6bfc15e2c0"
-version = "0.1.8"
+version = "0.1.9"
 
 [deps]
 InverseFunctions = "3587e190-3f89-42d0-90ee-14403ec27112"
@@ -9,19 +9,23 @@ Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
 
 [weakdeps]
 InverseFunctions = "3587e190-3f89-42d0-90ee-14403ec27112"
+Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
 
 [extensions]
 ChangesOfVariablesInverseFunctionsExt = "InverseFunctions"
+ChangesOfVariablesTestExt = "Test"
 
 [compat]
 InverseFunctions = "0.1"
 LinearAlgebra = "<0.0.1, 1"
+Test = "<0.0.1, 1"
 julia = "1"
 
 [extras]
 Documenter = "e30172f5-a6a5-5a46-863b-614d45cd2de4"
 ForwardDiff = "f6369f11-7733-5829-9624-2563aa707210"
 InverseFunctions = "3587e190-3f89-42d0-90ee-14403ec27112"
+Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
 
 [targets]
-test = ["Documenter", "InverseFunctions", "ForwardDiff"]
+test = ["Documenter", "InverseFunctions", "ForwardDiff", "Test"]

README.md

@@ -12,8 +12,8 @@ changes for functions that perform a change of variables (like coordinate
 transformations).
 
 `ChangesOfVariables` is a very lightweight package and has no dependencies
-beyond `Base`, `LinearAlgebra` and `Test` (plus a weak depdendency on
-`InverseFunctions`).
+beyond `Base` and `LinearAlgebra` (plus a weak dependency on `InverseFunctions`
+and `Test`).
 
 ## Documentation
 

ext/ChangesOfVariablesTestExt.jl

@@ -0,0 +1,31 @@
+module ChangesOfVariablesTestExt
+
+using Test: @inferred, @test, @testset
+using ChangesOfVariables: ChangesOfVariables, logabsdet, with_logabsdet_jacobian
+
+function ChangesOfVariables.test_with_logabsdet_jacobian(f, x, getjacobian; compare = isapprox, kwargs...)
+    @testset "test_with_logabsdet_jacobian: $f with input $x" begin
+        ref_y, test_type_inference = try
+            @inferred(f(x)), true
+        catch err
+            f(x), false
+        end
+
+        y, ladj = if test_type_inference
+            @inferred with_logabsdet_jacobian(f, x)
+        else
+            with_logabsdet_jacobian(f, x)
+        end
+
+        ref_ladj = _generalized_logabsdet(getjacobian(f, x))[1]
+
+        @test compare(y, ref_y; kwargs...)
+        @test compare(ladj, ref_ladj; kwargs...)
+    end
+    return nothing
+end
+
+_generalized_logabsdet(A) = logabsdet(A)
+_generalized_logabsdet(x::Real) = log(abs(x))
+
+end # module

src/ChangesOfVariables.jl

@@ -10,14 +10,51 @@ transformations).
 module ChangesOfVariables
 
 using LinearAlgebra
-using Test
 
 include("with_ladj.jl")
 include("setladj.jl")
-include("test.jl")
+
+"""
+    ChangesOfVariables.test_with_logabsdet_jacobian(f, x, getjacobian; compare = isapprox, kwargs...)
+
+Test if [`with_logabsdet_jacobian(f, x)`](@ref) is implemented correctly.
+
+Checks if the result of `with_logabsdet_jacobian(f, x)` is approximately
+equal to `(f(x), logabsdet(getjacobian(f, x)))`
+
+So the test uses `getjacobian(f, x)` to calculate a reference Jacobian for
+`f` at `x`. Passing `ForwardDiff.jabobian`, `Zygote.jacobian` or similar as
+the `getjacobian` function will do fine in most cases. If input and output
+of `f` are real scalar values, use `ForwardDiff.derivative`.
+
+Note that the result of `getjacobian(f, x)` must be a real-valued matrix
+or a real scalar, so you may need to use a custom `getjacobian` function
+that transforms the shape of `x` and `f(x)` internally, in conjunction
+with automatic differentiation.
+
+`kwargs...` are forwarded to `compare`.
+
+!!! Note
+    On Julia >= 1.9, you have to load the `Test` standard library to be able to use
+    this function.
+"""
+function test_with_logabsdet_jacobian end
 
 @static if !isdefined(Base, :get_extension)
     include("../ext/ChangesOfVariablesInverseFunctionsExt.jl")
+    include("../ext/ChangesOfVariablesTestExt.jl")
+end
+
+# Better error message if users forget to load Test
+if isdefined(Base, :get_extension) && isdefined(Base.Experimental, :register_error_hint)
+    function __init__()
+        Base.Experimental.register_error_hint(MethodError) do io, exc, _, _
+            if exc.f === test_with_logabsdet_jacobian &&
+                (Base.get_extension(ChangesOfVariables, :ChangesOfVariablesTest) === nothing)
+                print(io, "\nDid you forget to load Test?")
+            end
+        end
+    end
 end
 
 end # module