Add display_results REPL debugging mode; improve error display

display_results(true) echoes both the incoming expression/function call
from Excel and the value returned to it in the Julia console (green,
via printstyled); display_results() reads the current setting back.
Guards the echo's own display(result) call so a broken show() method
can't prevent a successful call's result from still reaching Excel.

Failed JuliaCall/JuliaEval calls now print in red instead of a plain
"====" divider, making them easier to spot in the console.

Also fixes a pre-existing test that hardcoded one particular Dict
iteration order, which isn't actually guaranteed by the language.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: PGS62

CHANGELOG.md

@@ -1,5 +1,12 @@
 # Changelog
 
+## [Unreleased]
+
+### New features
+
+- New `display_results` function: switch it on to echo, in the Julia console, both the expression or function call arriving from Excel and the value being returned to it - useful for seeing exactly what `JuliaCall`/`JuliaEval` are doing without leaving Excel.
+- Clearer Julia console output when a `JuliaCall`/`JuliaEval` call fails: the error and the expression that caused it are now shown in colour, making them easier to spot among other console output.
+
 ## [2.0.0] - 2026-08-19
 
 ### Breaking changes

src/JuliaExcel.jl

@@ -1,5 +1,5 @@
 module JuliaExcel
-export start_server, setxlpid, getcommsfolder, ExcelError
+export start_server, setxlpid, getcommsfolder, ExcelError, display_results, args_from_xl
 
 using DataFrames: DataFrames, DataFrame, Missing
 using Dates: Dates, Date, DateTime
@@ -9,6 +9,7 @@ using Sockets: Sockets
 const global xlpid = Ref(0)
 const global commsfolder = Ref("")
 const global xlport = Ref(0)
+const global _display_results = Ref(false)
 
 """
     ExcelError(code::Int)

src/comms.jl

@@ -19,6 +19,22 @@ function getxlpid()
     xlpid[]
 end
 
+"""
+    display_results(switch::Bool)
+Switch on or off display in the REPL of both the incoming expression/function call from Excel
+and the value returned to Excel, for calls via JuliaCall and JuliaEval.
+"""
+function display_results(switch::Bool)
+    _display_results[] = switch
+    "Results from JuliaCall/JuliaEval $(switch ? "will" : "will not") display in REPL"
+end
+
+"""
+    display_results()
+Returns whether display in the REPL of results of calls from Excel is currently switched on.
+"""
+display_results() = _display_results[]
+
 """
     getcommsfolder()
 Returns the name of the comms folder used by JuliaExcel. See also `setcommsfolder`.
@@ -101,18 +117,28 @@ Called by the HTTP request handler in `start_server` for requests to `/eval`, or
 from VBA calls to JuliaEval and JuliaEvalVBA.
 """
 function srv_eval_inner(expression::String)::String
+    if _display_results[]
+        printstyled("from_xl> ", color=:green)
+        println(expression)
+    end
+    success = true
     global result = try
         Main.eval(Meta.parse(expression))
     catch e
-        println("="^100)
-        if length(expression) > 500
-            println("Something went wrong evaluating the contents of an expression")
-        else
-            println("Something went wrong evaluating the expression:")
-            println(expression)
-        end
+        success = false
+        printstyled("Something went wrong evaluating the expression: ", color=:red)
+        println(expression)
         friendly_error(e)
     end
+    if _display_results[] && success
+        printstyled("to_xl> ", color=:green)
+        try
+            display(result)
+        catch e
+            printstyled("(could not display result of type $(typeof(result)): $e)\n", color=:red)
+        end
+        println("")
+    end
     Base.invokelatest(_encode_result_for_xl, result)
 end
 
@@ -132,11 +158,11 @@ without this cap a single error could still flood the cell. "Julia REPL has more
 stacktrace!" points the user at where the full detail actually lives.
 """
 function friendly_error(e)
+    print("\n")
     showerror(stdout, e, catch_backtrace())
-    println("")
-    println("="^100)
     io = IOBuffer()
     showerror(io, e)
+    print("\n\n")
     lines = split(String(take!(io)), '\n')
     error_desc = join(first(lines, 2), ' ')
     if length(error_desc) > 500
@@ -166,20 +192,37 @@ function srv_call_inner(payload::String)::String
     fn_name = "<unknown>"
     global args_from_xl = ["<unknown>"]
     broadcasting = false
+    success = true
     global result = try
         decoded = decode_from_xl(payload)
         fn_name = decoded[1]::String
+        if _display_results[]
+            expression = "$fn_name(args_from_xl...)"
+            printstyled("from_xl> ", color=:green)
+            println(expression)
+        end
         broadcasting = endswith(fn_name, ".")
         broadcasting && (fn_name = chop(fn_name))
         fn_to_call = Main.eval(Meta.parse(fn_name))             # fast: parses only the short function name
         args_from_xl = decoded[2:end]
         broadcasting ? broadcast(fn_to_call, args_from_xl...) : fn_to_call(args_from_xl...)
     catch e
-        println("="^100)
-        call_desc = broadcasting ? "$fn_name.(JuliaExcel.args_from_xl...)" : "$fn_name(JuliaExcel.args_from_xl...)"
-        println("Something went wrong calling the Julia function $fn_name from Excel, against arguments saved in JuliaExcel.args_from_xl (until overwritten by the next call), so the error should be reproducible from here with '$call_desc'.")
+        success = false
+        call_desc = broadcasting ? "$fn_name.(args_from_xl...)" : "$fn_name(args_from_xl...)"
+        printstyled("Something went wrong calling the Julia function $fn_name", color=:red)
+        print(" from Excel against\narguments saved in args_from_xl (overwritten by the next call),")
+        print(" so\nthe error should be reproducible from here with '$call_desc'.\n\n")
         friendly_error(e)
     end
+    if _display_results[] && success
+        printstyled("to_xl> ", color=:green)
+        try
+            display(result)
+        catch e
+            printstyled("(could not display result of type $(typeof(result)): $e)\n", color=:red)
+        end
+        println("")
+    end
     Base.invokelatest(_encode_result_for_xl, result)
 end
 

test/runtests.jl

@@ -44,7 +44,10 @@ end
     @test JuliaExcel.encode_for_xl(Any[1, 2, 3.0, π]) == "*1,4;2,2,17,17,;^1^2#4008000000000000#400921FB54442D18"
     @test JuliaExcel.encode_for_xl([1, true, "x"]) == "*1,3;2,1,2,;^1T£x"
     @test JuliaExcel.encode_for_xl([1, [2, 3]]) == "*1,2;2,14,;^1*1,2;2,2,;^2^3"
-    @test JuliaExcel.encode_for_xl(Dict("a"=>1, "b"=>2)) == "H2;2,2,2,2,;£b^2£a^1"
+    # Dict key order isn't guaranteed by the language, and can differ between Julia versions/hash
+    # seeds - accept either encoding order rather than hardcoding one.
+    @test JuliaExcel.encode_for_xl(Dict("a" => 1, "b" => 2)) in
+          ("H2;2,2,2,2,;£a^1£b^2", "H2;2,2,2,2,;£b^2£a^1")
 
     @test round_trip(1)
     @test round_trip(1.0)
@@ -231,6 +234,14 @@ end
                      "Julia REPL has more details and stacktrace!"
     end
 
+    # display_results (comms.jl) - toggles REPL echoing of calls from Excel; check both the
+    # setter's return-value contract and the getter round-trips it correctly. Ends on false (the
+    # default) so it doesn't leak into any test that follows.
+    @test JuliaExcel.display_results(true) == "Results from JuliaCall/JuliaEval will display in REPL"
+    @test JuliaExcel.display_results() == true
+    @test JuliaExcel.display_results(false) == "Results from JuliaCall/JuliaEval will not display in REPL"
+    @test JuliaExcel.display_results() == false
+
 end
 
 # The VBA-side test suite (modTest.RunTests) needs a live Excel/VBA session (via COM automation),