From 87f8a85a878c0cc0226cd57ff8ee32677c95c1b4 Mon Sep 17 00:00:00 2001
From: Yutaka Ichibangase <yichiban@gmail.com>
Date: Wed, 12 Jun 2024 21:53:17 +0900
Subject: [PATCH] fix variable's WriteTerm() when following/followed by a
 letter-digit operator.

---
 engine/variable.go      | 16 +++++++++++++---
 engine/variable_test.go |  2 ++
 interpreter_test.go     |  3 +++
 3 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/engine/variable.go b/engine/variable.go
index 994289c7..a93380ed 100644
--- a/engine/variable.go
+++ b/engine/variable.go
@@ -28,11 +28,21 @@ func (v Variable) WriteTerm(w io.Writer, opts *WriteOptions, env *Env) error {
 		return x.WriteTerm(w, opts, env)
 	}
 
+	ew := errWriter{w: w}
+
+	if letterDigit(opts.left.name) {
+		_, _ = ew.Write([]byte(" "))
+	}
 	if a, ok := opts.variableNames[v]; ok {
-		return a.WriteTerm(w, opts.withQuoted(false).withLeft(operator{}).withRight(operator{}), env)
+		_ = a.WriteTerm(&ew, opts.withQuoted(false).withLeft(operator{}).withRight(operator{}), env)
+	} else {
+		_, _ = ew.Write([]byte(fmt.Sprintf("_%d", v)))
 	}
-	_, err := w.Write([]byte(fmt.Sprintf("_%d", v)))
-	return err
+	if letterDigit(opts.right.name) {
+		_, _ = ew.Write([]byte(" "))
+	}
+
+	return ew.err
 }
 
 func (v Variable) Compare(t Term, env *Env) int {
diff --git a/engine/variable_test.go b/engine/variable_test.go
index 3692d40f..90ca997c 100644
--- a/engine/variable_test.go
+++ b/engine/variable_test.go
@@ -21,6 +21,8 @@ func TestVariable_WriteTerm(t *testing.T) {
 	}{
 		{title: "unnamed", v: x, output: fmt.Sprintf("_%d", x)},
 		{title: "variable_names", v: x, opts: WriteOptions{variableNames: map[Variable]Atom{x: NewAtom("Foo")}}, output: `Foo`},
+		{title: "following a letter-digit operator", v: x, opts: WriteOptions{left: operator{name: NewAtom("is")}}, output: fmt.Sprintf(" _%d", x)},
+		{title: "followed by a letter-digit operator", v: x, opts: WriteOptions{right: operator{name: NewAtom("is")}}, output: fmt.Sprintf("_%d ", x)},
 	}
 
 	var buf bytes.Buffer
diff --git a/interpreter_test.go b/interpreter_test.go
index 0784ba79..a820e584 100644
--- a/interpreter_test.go
+++ b/interpreter_test.go
@@ -308,8 +308,11 @@ func TestNew_variableNames(t *testing.T) {
 		{name: "55", query: `S=1+T,N=' /*r*/V',write_term(S,[quoted(true), variable_names([N=T])]).`, output: `1+ /*r*/V`},
 		{name: "58", query: `S=1+T,N=(+),write_term(S,[quoted(true), variable_names([N=T])]).`, output: `1++`},
 		{name: "59", query: `S=T+1,N=(+),write_term(S,[quoted(true), variable_names([N=T])]).`, output: `++1`},
+		{name: "73", query: `S=(1 is T),N='X',write_term(S,[quoted(true), variable_names([N=T])]).`, output: `1 is X`},
 		{name: "69", query: `read_term(T, [singletons(1)]).`, waits: true},
 		{name: "70", input: `a.`, query: `\+read_term(T, [singletons(1)]).`},
+		{name: "71", query: `write_term(T,[variable_names(['Bad'=T]),variable_names(['Good'=T])]).`, output: `Good`},
+		{name: "72", query: `read_term(T,[singletons([])]).`, waits: true},
 	}
 
 	for _, tt := range tests {