From 581f774284fa1b833255218165f4c958fe741500 Mon Sep 17 00:00:00 2001
From: Alexander Bantyev <balsoft@balsoft.ru>
Date: Tue, 28 Dec 2021 15:53:21 +0300
Subject: [PATCH 1/3] BaseError::calcWhat: take loggerSettings.showTrace into
 account

Text representation for errors should include the trace if
--show-trace is passed.
---
 src/libutil/error.cc | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/libutil/error.cc b/src/libutil/error.cc
index 203d79087..dd9678ee7 100644
--- a/src/libutil/error.cc
+++ b/src/libutil/error.cc
@@ -25,7 +25,7 @@ const string & BaseError::calcWhat() const
         err.name = sname();
 
         std::ostringstream oss;
-        showErrorInfo(oss, err, false);
+        showErrorInfo(oss, err, loggerSettings.showTrace);
         what_ = oss.str();
 
         return *what_;

From 2dc29e0d932822baa50dbc3e81ae108382c39062 Mon Sep 17 00:00:00 2001
From: Alexander Bantyev <balsoft@balsoft.ru>
Date: Tue, 28 Dec 2021 15:54:46 +0300
Subject: [PATCH 2/3] Add a test that nix repl --show-trace actually shows the
 trace

---
 tests/repl.sh                | 10 +++++++++-
 tests/undefined-variable.nix |  1 +
 2 files changed, 10 insertions(+), 1 deletion(-)
 create mode 100644 tests/undefined-variable.nix

diff --git a/tests/repl.sh b/tests/repl.sh
index 995db869c..0e23a98db 100644
--- a/tests/repl.sh
+++ b/tests/repl.sh
@@ -13,6 +13,10 @@ failing = import ./simple-failing.nix
 :log failing
 "
 
+replUndefinedVariable="
+import ./undefined-variable.nix
+"
+
 testRepl () {
     local nixArgs=("$@")
     local replOutput="$(nix repl "${nixArgs[@]}" <<< "$replCmds")"
@@ -22,10 +26,14 @@ testRepl () {
     nix path-info "${nixArgs[@]}" "$outPath"
     # simple.nix prints a PATH during build
     echo "$replOutput" | grep -qs 'PATH=' || fail "nix repl :log doesn't output logs"
-    local replOutput="$(nix repl "${nixArgs[@]}" <<< "$replFailingCmds")"
+    local replOutput="$(nix repl "${nixArgs[@]}" <<< "$replFailingCmds" 2>&1)"
     echo "$replOutput"
     echo "$replOutput" | grep -qs 'This should fail' \
       || fail "nix repl :log doesn't output logs for a failed derivation"
+    local replOutput="$(nix repl --show-trace "${nixArgs[@]}" <<< "$replUndefinedVariable" 2>&1)"
+    echo "$replOutput"
+    echo "$replOutput" | grep -qs "while evaluating the file" \
+      || fail "nix repl --show-trace doesn't show the trace"
 }
 
 # Simple test, try building a drv
diff --git a/tests/undefined-variable.nix b/tests/undefined-variable.nix
new file mode 100644
index 000000000..579985497
--- /dev/null
+++ b/tests/undefined-variable.nix
@@ -0,0 +1 @@
+let f = builtins.toFile "test-file.nix" "asd"; in import f

From a26351da02689d1e3ee562a6d89d93c12816e476 Mon Sep 17 00:00:00 2001
From: Alexander Bantyev <balsoft@balsoft.ru>
Date: Tue, 28 Dec 2021 16:13:07 +0300
Subject: [PATCH 3/3] Add ability to toggle show-trace from within the repl

---
 doc/manual/src/release-notes/rl-next.md |  2 ++
 src/nix/repl.cc                         | 15 ++++++++++++++-
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/doc/manual/src/release-notes/rl-next.md b/doc/manual/src/release-notes/rl-next.md
index ac662c132..2a67a39b1 100644
--- a/doc/manual/src/release-notes/rl-next.md
+++ b/doc/manual/src/release-notes/rl-next.md
@@ -2,3 +2,5 @@
 
 * The TOML parser used by `builtins.fromTOML` has been replaced by [a
   more compliant one](https://github.com/ToruNiina/toml11).
+* Added `:st`/`:show-trace` commands to nix repl, which are used to
+  set or toggle display of error traces.
diff --git a/src/nix/repl.cc b/src/nix/repl.cc
index 39a5a31de..a62ad66c2 100644
--- a/src/nix/repl.cc
+++ b/src/nix/repl.cc
@@ -430,7 +430,8 @@ bool NixRepl::processLine(string line)
              << "  :t <expr>     Describe result of evaluation\n"
              << "  :u <expr>     Build derivation, then start nix-shell\n"
              << "  :doc <expr>   Show documentation of a builtin function\n"
-             << "  :log <expr>   Show logs for a derivation\n";
+             << "  :log <expr>   Show logs for a derivation\n"
+             << "  :st [bool]    Enable, disable or toggle showing traces for errors\n";
     }
 
     else if (command == ":a" || command == ":add") {
@@ -572,6 +573,18 @@ bool NixRepl::processLine(string line)
             throw Error("value does not have documentation");
     }
 
+    else if (command == ":st" || command == ":show-trace") {
+        if (arg == "false" || (arg == "" && loggerSettings.showTrace)) {
+            std::cout << "not showing error traces\n";
+            loggerSettings.showTrace = false;
+        } else if (arg == "true" || (arg == "" && !loggerSettings.showTrace)) {
+            std::cout << "showing error traces\n";
+            loggerSettings.showTrace = true;
+        } else {
+            throw Error("unexpected argument '%s' to %s", arg, command);
+        };
+    }
+
     else if (command != "")
         throw Error("unknown command '%1%'", command);