Improve the error message for constructors (#254)

* show Type{Foo} rather than DataType in the rrule MethodError

* add rule to the docs

Author: mzgubic

docs/src/index.md

@@ -117,7 +117,7 @@ test_scalar: relu at -0.5 |   11     11
 
 ## Testing constructors and functors (callable objects)
 
-Testing constructor and functors works as you would expect. For struct `Foo`
+Testing constructor and functors works as you would expect. For struct `Foo`,
 ```julia
 struct Foo
     a::Float64
@@ -127,7 +127,27 @@ Base.length(::Foo) = 1
 Base.iterate(f::Foo) = iterate(f.a)
 Base.iterate(f::Foo, state) = iterate(f.a, state)
 ```
-the `f/rrule`s can be tested by
+
+after defining the constructor and functor `f/rule`s,
+
+```julia
+function ChainRulesCore.rrule(::Type{Foo}, val) # constructor rrule
+    y = Foo(val)
+    Foo_pb(ΔFoo) = (NoTangent(), unthunk(ΔFoo).a)
+    return y, Foo_pb
+end
+
+function ChainRulesCore.rrule(foo::Foo, val) # functor rrule
+    y = foo(val)
+    function foo_pb(Δ)
+        Δut = unthunk(Δ)
+        return (Tangent{Foo}(;a=Δut), Δut)
+    end
+    return y, foo_pb
+end
+```
+
+both `f/rrule`s can be tested by
 ```julia
 test_rrule(Foo, rand()) # constructor
 

src/testers.jl

@@ -125,7 +125,7 @@ function test_frule(
         end
 
         res = call_on_copy(frule_f, config, tangents, primals...)
-        res === nothing && throw(MethodError(frule_f, typeof(primals)))
+        res === nothing && throw(MethodError(frule_f, Tuple{Core.Typeof.(primals)...}))
         @test_msg "The frule should return (y, ∂y), not $res." res isa Tuple{Any,Any}
         Ω_ad, dΩ_ad = res
         Ω = call_on_copy(primals...)
@@ -201,7 +201,7 @@ function test_rrule(
             _test_inferred(rrule_f, config, primals...; fkwargs...)
         end
         res = rrule_f(config, primals...; fkwargs...)
-        res === nothing && throw(MethodError(rrule_f, typeof(primals)))
+        res === nothing && throw(MethodError(rrule_f, Tuple{Core.Typeof.(primals)...}))
         y_ad, pullback = res
         y = call(primals...)
         test_approx(y_ad, y; isapprox_kwargs...)  # make sure primal is correct