let code_string return nothing if definition returns nothing (#92)

As suggested by @KristofferC in JuliaDocs/Documenter.jl#1779 (comment). This is related to problems using CodeTracking.jl with Documenter.jl (see JuliaDocs/Documenter.jl#1779) or Jupyter notebooks (see #51). Instead of throwing an exception, `code_string` now returns `nothing` if `definition` returns `nothing`. This makes it easier to handle such a problematic case by the user.

Co-authored-by: Kristoffer Carlsson <kristoffer.carlsson@juliacomputing.com>
Co-authored-by: Tim Holy <tim.holy@gmail.com>

Author: 3 people

src/CodeTracking.jl

@@ -296,7 +296,10 @@ end
 
 Returns the code-string for the method definition for `f` with the specified types.
 """
-code_string(f, t) = definition(String, which(f, t))[1]
+function code_string(f, t)
+    def = definition(String, which(f, t))
+    return def === nothing ? nothing : def[1]
+end
 macro code_string(ex0...)
     InteractiveUtils.gen_call_with_extracted_types_and_kwargs(__module__, :code_string, ex0)
 end

test/runtests.jl

@@ -148,6 +148,13 @@ isdefined(Main, :Revise) ? Main.Revise.includet("script.jl") : include("script.j
         m = first(methods(getfield(mod, :eval)))
         @test definition(String, m) === nothing
     end
+
+    # Related to issue [#51](https://github.com/timholy/CodeTracking.jl/issues/51)
+    # and https://github.com/JuliaDocs/Documenter.jl/issues/1779
+    ex = :(f_no_linenum(::Int) = 1)
+    deleteat!(ex.args[2].args, 1)    # delete the file & line number info
+    eval(ex)
+    @test code_string(f_no_linenum, (Int,)) === nothing
 end
 
 @testset "With Revise" begin