Updated the project structure, and modified the Makefile to hopefully be future proof so I can freely make sub-folders in the src/ folder and not have to worry about breaking the Makefile.

.gitignore

@@ -1,3 +1,4 @@
 clox
 *.lox
 obj/
+bin/

Makefile

@@ -1,17 +1,26 @@
 CC = gcc
-SOURCEFILES := $(wildcard *.c)
+CFLAGS=-Wall
+SRC=src
+OBJ=obj
+SRCS=$(wildcard $(SRC)/*.c)
+OBJS=$(patsubst $(SRC)/%.c, $(OBJ)/%.o, $(SRCS))
 
-SOURCE := $(wildcard *.c)
+BINDIR=bin
+BIN=$(BINDIR)/clox
 
-$(SOURCE:.c=.o): $(SOURCE)
-	@mkdir -p obj
-	$(CC) $^ -o obj/clox
+all:$(BIN)
+
+$(BIN): $(OBJS)
+	$(CC) $(CFLAGS) $(OBJS) -o $@
+
+$(OBJ)/%.o: $(SRC)/%.c
+	$(CC) $(CFLAGS) -c $< -o $@
 
 .PHONY: clean
 .PHONY: test
 
 clean:
-	rm -r obj
+	rm -r $(BINDIR)/* $(OBJ)/*
 
 test:
-	./obj/clox test.lox
+	$(BIN) test.lox

expr.c

@@ -1,18 +0,0 @@
-#include "expr.h"
-
-char* Parenthesize(char*, int, ...);
-
-void VisitBinary(struct binary expr) {
-
-}
-
-void VisitGrouping(struct grouping);
-void VisitLiteral(Token);
-void VisitUnary(struct unary);
-
-char* Parenthesize(char* operator, int count, ...) {
-    va_list list;
-    va_start(list, count);
-
-    va_arg(list, Expr);
-}

src/expr.h

@@ -40,9 +40,9 @@ struct Expr {
     } expression;
 };
 
-void VisitBinary(struct binary);
-void VisitGrouping(struct grouping);
-void VisitLiteral(Token);
-void VisitUnary(struct unary);
+// void VisitBinary(struct binary);
+// void VisitGrouping(struct grouping);
+// void VisitLiteral(Token);
+// void VisitUnary(struct unary);
 
 #endif