add simple REPL

Makefile

@@ -4,7 +4,7 @@ LFLAGS = -lm
 
 .PHONY = all clean
 
-all: smoke smoke_pr bench example example2 example3
+all: smoke smoke_pr repl bench example example2 example3
 
 
 smoke: smoke.c tinyexpr.c
@@ -15,6 +15,9 @@ smoke_pr: smoke.c tinyexpr.c
 	$(CC) $(CCFLAGS) -DTE_POW_FROM_RIGHT -DTE_NAT_LOG -o $@ $^ $(LFLAGS)
 	./$@
 
+repl: repl.o tinyexpr.o
+	$(CC) $(CCFLAGS) -o $@ $^ $(LFLAGS) -lreadline
+
 bench: benchmark.o tinyexpr.o
 	$(CC) $(CCFLAGS) -o $@ $^ $(LFLAGS)
 
@@ -31,4 +34,4 @@ example3: example3.o tinyexpr.o
 	$(CC) -c $(CCFLAGS) $< -o $@
 
 clean:
-	rm -f *.o *.exe example example2 example3 bench smoke_pr smoke
+	rm -f *.o *.exe example example2 example3 bench repl smoke_pr smoke

repl.c

@@ -0,0 +1,52 @@
+#include "tinyexpr.h"
+#include <stdlib.h>
+#include <stdio.h>
+#include <readline/readline.h>
+#include <readline/history.h>
+
+static int eval(const char *str) {
+    int err = 0;
+    double r = te_interp(str, &err);
+    if (err != 0) {
+        printf("Error at position %i\n", err);
+        return -1;
+    } else {
+        printf("%g\n", r);
+        return 0;
+    }
+}
+
+static void repl() {
+    while (1) {
+        char *line = readline("> ");
+        if (line == NULL) {
+            break;
+        } else if (strcmp(line, "q") == 0 || strcmp(line, "quit") == 0) {
+            free(line);
+            break;
+        }
+
+        if (eval(line) != -1) {
+            add_history(line);
+        }
+
+        free(line);
+    }
+}
+
+int main(int argc, char **argv) {
+    if (argc == 3 && strcmp(argv[1], "-e") == 0) {
+        if (eval(argv[2]) == -1) {
+            return 1;
+        } else {
+            return 0;
+        }
+    } else if (argc == 1) {
+        repl();
+        return 0;
+    } else {
+        printf("Usage: %s\n", argv[0]);
+        printf("       %s -e <expression>\n", argv[0]);
+        return 1;
+    }
+}