Merge pull request #58 from JuliaDiff/ox/thunkwhere

Record where thunks actually created

Author: oxinabox

src/differentials.jl

@@ -212,8 +212,17 @@ struct Thunk{F} <: AbstractThunk
     f::F
 end
 
+
+"""
+    @thunk expr
+
+Define a [`Thunk`](@ref) wrapping the `expr`, to lazily defer its evaluation.
+"""
 macro thunk(body)
-    return :(Thunk(() -> $(esc(body))))
+    # Basically `:(Thunk(() -> $(esc(body))))` but use the location where it is defined.
+    # so we get useful stack traces if it errors.
+    func = Expr(:->, Expr(:tuple), Expr(:block, __source__, body))
+    return :(Thunk($(esc(func))))
 end
 
 # have to define this here after `@thunk` and `Thunk` is defined

test/differentials.jl

@@ -66,6 +66,21 @@
             @test (@thunk(3))() == 3
             @test (@thunk(@thunk(3)))() isa Thunk
         end
+
+        @testset "erroring thunks should include the source in the backtrack" begin
+            expected_line = (@__LINE__) + 2  # for testing it is at right palce
+            try
+                x = @thunk(error())
+                extern(x)
+            catch err
+                err isa ErrorException || rethrow()
+                st = stacktrace(catch_backtrace())
+                # Should be 2nd last line, as last line will be the `error` function
+                stackframe = st[2]
+                @test stackframe.line == expected_line
+                @test stackframe.file == Symbol(@__FILE__)
+            end
+        end
     end
 
     @testset "No ambiguities in $f" for f in (+, *)