From ff0fd91ed23ae9d851bf27c4df3ec77f7028699b Mon Sep 17 00:00:00 2001
From: pennae <github@quasiparticle.net>
Date: Sat, 5 Mar 2022 14:20:25 +0100
Subject: [PATCH] remove Symbol::empty

the only use of this function is to determine whether a lambda has a non-set
formal, but this use is arguably better served by Symbol::set and using a
non-Symbol instead of an empty symbol in the parser when no such formal is present.
---
 src/libexpr/eval.cc         | 4 ++--
 src/libexpr/nixexpr.cc      | 8 ++++----
 src/libexpr/parser.y        | 2 +-
 src/libexpr/symbol-table.hh | 5 -----
 src/libexpr/value-to-xml.cc | 2 +-
 5 files changed, 8 insertions(+), 13 deletions(-)

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index a5eb2e473..b39b24227 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -1342,7 +1342,7 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
             ExprLambda & lambda(*vCur.lambda.fun);
 
             auto size =
-                (lambda.arg.empty() ? 0 : 1) +
+                (!lambda.arg.set() ? 0 : 1) +
                 (lambda.hasFormals() ? lambda.formals->formals.size() : 0);
             Env & env2(allocEnv(size));
             env2.up = vCur.lambda.env;
@@ -1355,7 +1355,7 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
             else {
                 forceAttrs(*args[0], pos);
 
-                if (!lambda.arg.empty())
+                if (lambda.arg.set())
                     env2.values[displ++] = args[0];
 
                 /* For each formal argument, get the actual argument.  If
diff --git a/src/libexpr/nixexpr.cc b/src/libexpr/nixexpr.cc
index a2def65a6..bf01935a9 100644
--- a/src/libexpr/nixexpr.cc
+++ b/src/libexpr/nixexpr.cc
@@ -144,9 +144,9 @@ void ExprLambda::show(std::ostream & str) const
             str << "...";
         }
         str << " }";
-        if (!arg.empty()) str << " @ ";
+        if (arg.set()) str << " @ ";
     }
-    if (!arg.empty()) str << arg;
+    if (arg.set()) str << arg;
     str << ": " << *body << ")";
 }
 
@@ -364,11 +364,11 @@ void ExprLambda::bindVars(const StaticEnv & env)
     StaticEnv newEnv(
         false, &env,
         (hasFormals() ? formals->formals.size() : 0) +
-        (arg.empty() ? 0 : 1));
+        (!arg.set() ? 0 : 1));
 
     Displacement displ = 0;
 
-    if (!arg.empty()) newEnv.vars.emplace_back(arg, displ++);
+    if (arg.set()) newEnv.vars.emplace_back(arg, displ++);
 
     if (hasFormals()) {
         for (auto & i : formals->formals)
diff --git a/src/libexpr/parser.y b/src/libexpr/parser.y
index 919b9cfae..49c401603 100644
--- a/src/libexpr/parser.y
+++ b/src/libexpr/parser.y
@@ -369,7 +369,7 @@ expr_function
   : ID ':' expr_function
     { $$ = new ExprLambda(CUR_POS, data->symbols.create($1), 0, $3); }
   | '{' formals '}' ':' expr_function
-    { $$ = new ExprLambda(CUR_POS, data->symbols.create(""), toFormals(*data, $2), $5); }
+    { $$ = new ExprLambda(CUR_POS, {}, toFormals(*data, $2), $5); }
   | '{' formals '}' '@' ID ':' expr_function
     {
       Symbol arg = data->symbols.create($5);
diff --git a/src/libexpr/symbol-table.hh b/src/libexpr/symbol-table.hh
index 48d20c29d..297605295 100644
--- a/src/libexpr/symbol-table.hh
+++ b/src/libexpr/symbol-table.hh
@@ -60,11 +60,6 @@ public:
         return s;
     }
 
-    bool empty() const
-    {
-        return s->empty();
-    }
-
     friend std::ostream & operator << (std::ostream & str, const Symbol & sym);
 };
 
diff --git a/src/libexpr/value-to-xml.cc b/src/libexpr/value-to-xml.cc
index afeaf5694..7f8edcba6 100644
--- a/src/libexpr/value-to-xml.cc
+++ b/src/libexpr/value-to-xml.cc
@@ -139,7 +139,7 @@ static void printValueAsXML(EvalState & state, bool strict, bool location,
 
             if (v.lambda.fun->hasFormals()) {
                 XMLAttrs attrs;
-                if (!v.lambda.fun->arg.empty()) attrs["name"] = v.lambda.fun->arg;
+                if (v.lambda.fun->arg.set()) attrs["name"] = v.lambda.fun->arg;
                 if (v.lambda.fun->formals->ellipsis) attrs["ellipsis"] = "1";
                 XMLOpenElement _(doc, "attrspat", attrs);
                 for (auto & i : v.lambda.fun->formals->lexicographicOrder())