[Ongoing] Rewrite frontend to use Flex/Bison

This is a merge of another experiment, so the changes are large:

- separated "modules" directory into frontend/backend
  - adjusted module names and moved files for this to happen
- removed modules lexer & parser
- removed all the unit tests (most were outdated)
- added Bison, flex, and C development tools to manifest.scm
- added lexer.l, a source file used by the flex utility with a functioning lexing implementation
- added parser.y, a source file used by the bison utility with a functioning parser implementation
- added node.c and node.h, which parser.y uses to construct an AST of a C source file (up to binary ops)
- added driver.c, a Guile-C interface that provides a module to scheme programs
- added a Makefile to make all of this
- added stuff to .gitignore

31 files changed, 584 insertions, 408 deletions

diff --git a/.dir-locals.el b/.dir-locals.el
index ba479e9..cf9d341 100644
--- a/.dir-locals.el
+++ b/.dir-locals.el
@@ -1,4 +1,5 @@
 ;;; Directory Local Variables            -*- no-byte-compile: t -*-
 ;;; For more information see (info "(emacs) Directory Variables")
 
-((scheme-mode . ((compile-command . "guix shell -m manifest.scm -- ./writing-a-c-compiler-tests/test_compiler ./src/ull --chapter 1 --stage lex"))))
+((scheme-mode . ((compile-command . "guix shell -m manifest.scm -- ./writing-a-c-compiler-tests/test_compiler ./ull --chapter 1 --stage lex")))
+ (c-mode . ((compile-command . "guix shell -m manifest.scm -- make clean all"))))
diff --git a/.gitignore b/.gitignore
index aff9071..c2b23bf 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,2 +1,8 @@
-/writing-a-c-compiler-tests/
-/*.log
+writing-a-c-compiler-tests/
+build/
+src/frontend/lexer.c
+src/frontend/lexer.h
+src/frontend/parser.c
+src/frontend/parser.h
+src/frontend/parser.output
+libull.so
\ No newline at end of file
diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..de737ad
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,34 @@
+CC = gcc
+CFLAGS = -fPIC -Wall -Wextra -Wpedantic `pkg-config --cflags guile-3.0`
+LIBS = `pkg-config --libs guile-3.0`
+
+TARGET ?= libull.so
+
+SRCDIR := src/frontend
+OBJDIR := build
+
+SRCS = $(SRCDIR)/lexer.c $(SRCDIR)/parser.c $(SRCDIR)/node.c $(SRCDIR)/driver.c
+OBJS = $(OBJDIR)/driver.o $(OBJDIR)/lexer.o $(OBJDIR)/parser.o $(OBJDIR)/node.o
+
+all: $(TARGET)
+
+$(TARGET): $(OBJS)
+	$(CC) -shared -o $@ $^ $(LIBS)
+
+$(OBJS): $(OBJDIR)/%.o : $(SRCDIR)/%.c $(SRCDIR)/lexer.h $(SRCDIR)/parser.h | $(OBJDIR)
+	$(CC) $(CFLAGS) -c $< -o $@
+
+$(SRCDIR)/lexer.h $(SRCDIR)/lexer.c: $(SRCDIR)/lexer.l
+	flex --header-file=$(SRCDIR)/lexer.h -o $(SRCDIR)/lexer.c $(SRCDIR)/lexer.l
+
+$(SRCDIR)/parser.h $(SRCDIR)/parser.c: $(SRCDIR)/parser.y
+	bison -d -v -o $(SRCDIR)/parser.c $(SRCDIR)/parser.y
+
+$(OBJDIR):
+	mkdir -p $(OBJDIR)
+
+clean:
+	rm -f $(OBJS) $(SRCDIR)/lexer.c $(SRCDIR)/lexer.h $(SRCDIR)/parser.c $(SRCDIR)/parser.h $(SRCDIR)/parser.output $(TARGET)
+
+
+.PHONY: all clean
diff --git a/manifest.scm b/manifest.scm
index 54ab185..d028b9c 100644
--- a/manifest.scm
+++ b/manifest.scm
@@ -2,17 +2,21 @@
              (gnu packages flex)
              (gnu packages commencement)
              (gnu packages base)
+             (gnu packages gdb)
              (gnu packages python)
              (gnu packages guile)
-	     (gnu packages pkg-config))
+             (gnu packages pkg-config)
+             (gnu packages valgrind))
 
 (packages->manifest
  (list
   bison
   flex
   gcc-toolchain
+  gdb
   gnu-make
   guile-next
   pkg-config
   python
+  valgrind
   ))
diff --git a/src/modules/ast/ir.scm b/src/backend/ast/ir.scm
index 50e9e29..102739c 100644
--- a/src/modules/ast/ir.scm
+++ b/src/backend/ast/ir.scm
@@ -1,4 +1,4 @@
-(define-module (ast ir)
+(define-module (backend ast ir)
   #:export (prog
             srout
             neg
diff --git a/src/backend/emitter/driver.scm b/src/backend/emitter/driver.scm
new file mode 100644
index 0000000..bd65d5a
--- /dev/null
+++ b/src/backend/emitter/driver.scm
@@ -0,0 +1,6 @@
+(define-module (backend emitter driver)
+  #:export (assembly->string))
+
+
+(define (assembly->string n)
+  (eval n (resolve-module '(backend emitter traverse))))
diff --git a/src/modules/emitter/traverse.scm b/src/backend/emitter/traverse.scm
index 6c0c19d..0a5b4cc 100644
--- a/src/modules/emitter/traverse.scm
+++ b/src/backend/emitter/traverse.scm
@@ -1,4 +1,4 @@
-(define-module (emitter traverse)
+(define-module (backend emitter traverse)
   #:export (prog
             subrout
             instr
diff --git a/src/modules/generator/allocate.scm b/src/backend/generator/allocate.scm
index f975fca..0d417db 100644
--- a/src/modules/generator/allocate.scm
+++ b/src/backend/generator/allocate.scm
@@ -1,5 +1,5 @@
-(define-module (generator allocate)
-  #:use-module (utils assign-stack)
+(define-module (backend generator allocate)
+  #:use-module (backend utils assign-stack)
   #:export (expansion->allocate))
 
 
diff --git a/src/modules/generator/assembly.scm b/src/backend/generator/assembly.scm
index 55eaaed..8a110f3 100644
--- a/src/modules/generator/assembly.scm
+++ b/src/backend/generator/assembly.scm
@@ -1,7 +1,7 @@
-(define-module (generator assembly)
-  #:use-module (ast ir)
-  #:use-module (utils assign-stack)
-  #:use-module (utils merge-instructions)
+(define-module (backend generator assembly)
+  #:use-module (backend ast ir)
+  #:use-module (backend utils assign-stack)
+  #:use-module (backend utils merge-instructions)
   #:export (instrs
             mov))
 
diff --git a/src/backend/generator/driver.scm b/src/backend/generator/driver.scm
new file mode 100644
index 0000000..6625388
--- /dev/null
+++ b/src/backend/generator/driver.scm
@@ -0,0 +1,9 @@
+(define-module (backend generator driver)
+  #:use-module (backend generator allocate)
+  #:export (tacky->assembly))
+
+
+(define (tacky->assembly n)
+  (eval (expansion->allocate
+         (eval n (resolve-module '(backend generator expansion))))
+        (resolve-module '(backend generator assembly))))
diff --git a/src/modules/generator/expansion.scm b/src/backend/generator/expansion.scm
index 5bfa878..9fe2e0a 100644
--- a/src/modules/generator/expansion.scm
+++ b/src/backend/generator/expansion.scm
@@ -1,6 +1,6 @@
-(define-module (generator expansion)
-  #:use-module (ast ir)
-  #:use-module (utils merge-instructions)
+(define-module (backend generator expansion)
+  #:use-module (backend ast ir)
+  #:use-module (backend utils merge-instructions)
   #:export (instrs
             not
             neg
diff --git a/src/backend/tacky/driver.scm b/src/backend/tacky/driver.scm
new file mode 100644
index 0000000..b9585c7
--- /dev/null
+++ b/src/backend/tacky/driver.scm
@@ -0,0 +1,6 @@
+(define-module (backend tacky driver)
+  #:export (ast->tacky))
+
+
+(define (ast->tacky n)
+  (eval n (resolve-module '(backend tacky traverse))))
diff --git a/src/modules/tacky/traverse.scm b/src/backend/tacky/traverse.scm
index 2fd4ca7..9a1b7aa 100644
--- a/src/modules/tacky/traverse.scm
+++ b/src/backend/tacky/traverse.scm
@@ -1,15 +1,11 @@
-(define-module (tacky traverse)
-  #:use-module (ast ir)
-  #:export (prog
-            func
+(define-module (backend tacky traverse)
+  #:use-module (backend ast ir)
+  #:export (func
             stmt
             unary
             const))
 
 
-(define (prog srout)
-  (list 'prog srout))
-
 (define (func name instrs)
   (list 'srout name instrs))
 
diff --git a/src/modules/utils/assign-stack.scm b/src/backend/utils/assign-stack.scm
index 1096846..dff9fc7 100644
--- a/src/modules/utils/assign-stack.scm
+++ b/src/backend/utils/assign-stack.scm
@@ -1,4 +1,4 @@
-(define-module (utils assign-stack)
+(define-module (backend utils assign-stack)
   #:export (make-location
             get-frame-size))
 
diff --git a/src/modules/utils/merge-instructions.scm b/src/backend/utils/merge-instructions.scm
index 754117c..f9fc043 100644
--- a/src/modules/utils/merge-instructions.scm
+++ b/src/backend/utils/merge-instructions.scm
@@ -1,4 +1,4 @@
-(define-module (utils merge-instructions)
+(define-module (backend utils merge-instructions)
   #:use-module (srfi srfi-1)
   #:export (merge-instr))
 
diff --git a/src/frontend/driver.c b/src/frontend/driver.c
new file mode 100644
index 0000000..52eb3f4
--- /dev/null
+++ b/src/frontend/driver.c
@@ -0,0 +1,88 @@
+#include "lexer.h"
+#include "parser.h"
+#include "node.h"
+#include <stdio.h>
+#include <libguile.h>
+
+/**
+ * @param node a tree of node structs representing a C source file.
+ * @return a scheme representation of node.
+ */
+SCM
+node_to_scm(Node *node) {
+  SCM ret = scm_list_1(scm_from_locale_symbol(node_types[node->type]));
+
+  SCM field_value = SCM_BOOL_F;
+  switch (node->type) {
+  case FUNC:
+    field_value = scm_from_locale_string(node->field.name);
+    break;
+  case EXPR:
+    field_value = scm_from_locale_symbol(node_ops[node->field.op]);
+    break;
+  case CONST:
+    field_value = scm_from_int32(node->field.val);
+    break;
+  default:
+    ;
+  }
+
+  if (scm_is_true(field_value))
+    ret = scm_append(scm_list_2(ret, scm_list_1(field_value)));
+
+  SCM child;
+  for (size_t i = 0; i < node->num_children; ++i) {
+    child = node_to_scm(node->children[i]);
+    ret = scm_append(scm_list_2(ret, scm_list_1(child)));
+  }
+
+  return ret;
+}
+
+/** \brief Parser driver for ull.
+ *         Given F, returns an AST of the program represented as a guile s-exp, or #f on a parse error.
+ *         if DO_PARSE is false, only performs lexing, returning #t on a success.
+ * @param f a preprocessed C file to parse.
+ * @param do_parse on false, do not perform the parsing stage.
+ * @return a SCM list representing the parse tree, or #t if do_parse is #f and lexing is successful, or #f otherwise.
+ */
+SCM
+file_to_ast_wrapper(SCM f, SCM do_parse)
+{
+  char *file = scm_to_locale_string(f);
+  Node *root = NULL;
+  SCM ret = SCM_BOOL_F;
+
+  yyin = fopen(file, "r");
+  if (yyin != NULL){
+    if (scm_is_true(do_parse)) {
+      if (yyparse(&root) == 0) {
+	ret = node_to_scm(root);
+	free_node(root);
+      }
+    } else {
+      ret = SCM_BOOL_T;
+      int token;
+      while ((token = yylex()) != 0) {
+	if (token == YYerror)
+	  ret = SCM_BOOL_F;
+      }
+    }
+  }
+
+  fclose(yyin);
+  free(file);
+  return ret;
+}
+
+void
+init_parser_driver()
+{
+  scm_c_define_gsubr("file->ast", 2, 0, 0, file_to_ast_wrapper);
+  scm_c_export("file->ast", NULL);
+}
+
+void
+scm_init_parser_driver_module() {
+  scm_c_define_module("frontend driver", init_parser_driver, NULL);
+}
diff --git a/src/frontend/driver.scm b/src/frontend/driver.scm
new file mode 100644
index 0000000..402e6e9
--- /dev/null
+++ b/src/frontend/driver.scm
@@ -0,0 +1,3 @@
+(define-module (frontend driver))
+
+(load-extension "./libull.so" "scm_init_parser_driver_module")
diff --git a/src/frontend/lexer.l b/src/frontend/lexer.l
new file mode 100644
index 0000000..5b72086
--- /dev/null
+++ b/src/frontend/lexer.l
@@ -0,0 +1,47 @@
+/*** definition section ***/
+%{
+  /* C to be copied verbatim */
+  #include "parser.h"
+%}
+
+/* read only one input file */
+%option noyywrap
+%option yylineno
+
+DIGIT    [0-9]
+ALPHA    [[:alpha:]_]
+
+/*** rules section ***/
+%%
+
+"("             {return L_PAREN;}
+")"             {return R_PAREN;}
+"{"             {return L_BRACK;}
+"}"             {return R_BRACK;}
+";"             {return SEMI_COL;}
+"~"             {return COMP;}
+"*"             {return MULT;}
+"/"             {return DIV;}
+"%"             {return MOD;}
+"-"             {return MINUS;}
+"+"             {return PLUS;}
+
+"int"           {return INT;}
+"void"          {return VOID;}
+"return"        {return RET;}
+
+{DIGIT}+        {yylval.ival = atol(yytext); return NUMBER;}
+{ALPHA}+        {
+  yylval.sval = strdup(yytext);
+  return WORD;
+       }
+
+[[:space:]]+    {/* discard */}
+.               {
+  printf("Error at line %d: unrecognized symbol \"%s\"\n", yylineno, yytext);
+  return YYerror;
+}
+
+%%
+
+/*** C section ***/
diff --git a/src/frontend/node.c b/src/frontend/node.c
new file mode 100644
index 0000000..344b48a
--- /dev/null
+++ b/src/frontend/node.c
@@ -0,0 +1,84 @@
+#include "node.h"
+#include <stddef.h>
+#include <stdlib.h>
+#include <string.h>
+#include <stdio.h>
+
+const char* node_types[] = {"prog", "func", "stmt", "expr", "const"};
+const char* node_ops[] = {"not", "neg", "plus", "minus", "mult", "div", "mod"};
+
+Node *create_node(enum node_type type) {
+  Node *node = malloc(sizeof(Node));
+  node->type = type;
+  node->children = NULL;
+  node->num_children = 0;
+  return node;
+}
+
+Node *create_function(char *name) {
+  Node *node = malloc(sizeof(Node));
+  node->type = FUNC;
+  node->field.name = name;
+  node->children = NULL;
+  node->num_children = 0;
+  return node;
+}
+
+Node *create_expr(enum node_op op) {
+  Node *node = malloc(sizeof(Node));
+  node->type = EXPR;
+  node->field.op = op;
+  node->children = NULL;
+  node->num_children = 0;
+  return node;
+}
+
+Node *create_const(int val) {
+  Node *node = malloc(sizeof(Node));
+  node->type = CONST;
+  node->field.val = val;
+  node->children = NULL;
+  node->num_children = 0;
+  return node;
+}
+
+void add_child(Node *parent, Node *child) {
+  size_t new_size = sizeof(Node*) * (parent->num_children + 1);
+  parent->children = realloc(parent->children, new_size);
+
+  parent->children[parent->num_children] = child;
+  parent->num_children++;
+}
+
+void free_node(Node *node) {
+  for (size_t i = 0; i < node->num_children; ++i) {
+    free_node(node->children[i]);
+  }
+  if (node->type == FUNC)
+    free(node->field.name);
+
+  free(node->children);
+  free(node);
+}
+
+void *print_node(Node *node, int indent) {
+  if (node == NULL) {
+    return;
+  }
+
+  for (int i = 0; i < indent; ++i)
+    printf(" ");
+
+  printf("type: %s", node_types[node->type]);
+  if (node->type == FUNC)
+    printf(", name: \"%s\"", node->field.name);
+  if (node->type == EXPR)
+    printf(", op: %s", node_ops[node->field.op]);
+  if (node->type == CONST)
+    printf(", val: %d", node->field.val);
+  printf("\n");
+
+  for (size_t i = 0; i < node->num_children; ++i) {
+    print_node(node->children[i], indent + 1);
+  }
+}
diff --git a/src/frontend/node.h b/src/frontend/node.h
new file mode 100644
index 0000000..71c8182
--- /dev/null
+++ b/src/frontend/node.h
@@ -0,0 +1,33 @@
+#ifndef NODE_H
+#define NODE_H
+
+#include <stddef.h>
+
+enum node_type{PROG, FUNC, STMT, EXPR, CONST};
+enum node_op{COMP_SYM, NEG_SYM, PLUS_SYM, MINUS_SYM, MULT_SYM, DIV_SYM, MOD_SYM};
+
+extern const char* node_types[];
+extern const char* node_ops[];
+
+typedef union {
+  char *name;
+  int val;
+  enum node_op op;
+} node_field;
+
+typedef struct Node {
+  enum node_type type;
+  node_field field;
+  struct Node **children;
+  size_t num_children;
+} Node;
+
+Node *create_node(enum node_type type);
+Node *create_function(char *name);
+Node *create_expr(enum node_op op);
+Node *create_const(int val);
+void add_child(Node *parent, Node *child);
+void free_node(Node *node);
+void *print_node(Node *node, int indent);
+
+#endif
diff --git a/src/frontend/parser.y b/src/frontend/parser.y
new file mode 100644
index 0000000..aa58f64
--- /dev/null
+++ b/src/frontend/parser.y
@@ -0,0 +1,116 @@
+/*** definition section ***/
+%{
+  /* C to be copied verbatim */
+  #include "lexer.h"
+  #include "node.h"
+  void yyerror(Node **root, const char *msg);
+%}
+
+%code requires {
+  #include "node.h"
+}
+
+%union {
+  Node *node;
+  int ival;
+  char *sval;
+};
+
+%locations
+%start input
+%parse-param {Node **root}
+
+%token L_PAREN R_PAREN L_BRACK R_BRACK SEMI_COL COMP MULT DIV MOD MINUS PLUS INT VOID RET
+%token <sval> WORD
+%token <ival> NUMBER
+
+%type <node> input
+%type <node> func
+%type <node> stmt
+%type <node> exp
+%type <node> term
+%type <node> factor
+%type <ival> un_op
+
+
+/*** rules section ***/
+%%
+input:   func {
+    *root = create_node(PROG);
+    add_child(*root, $1);
+  }
+;
+
+func:   INT WORD L_PAREN VOID R_PAREN L_BRACK stmt R_BRACK {
+    $$ = create_function($2);
+    add_child($$, $7);
+  }
+;
+
+stmt:   RET exp SEMI_COL {
+    $$ = create_node(STMT);
+    add_child($$, $2);
+  }
+;
+
+exp:    term {
+    $$ = $1;
+  }
+|       exp PLUS term {
+    $$ = create_expr(PLUS_SYM);
+    add_child($$, $1);
+    add_child($$, $3);
+  }
+|       exp MINUS term {
+    $$ = create_expr(MINUS_SYM);
+    add_child($$, $1);
+    add_child($$, $3);
+  }
+|       un_op exp {
+    $$ = create_expr($1);
+    add_child($$, $2);
+  }
+;
+
+term:   factor {
+    $$ = $1;
+  }
+|       term MULT factor {
+    $$ = create_expr(MULT_SYM);
+    add_child($$, $1);
+    add_child($$, $3);
+  }
+|       term DIV factor {
+    $$ = create_expr(DIV_SYM);
+    add_child($$, $1);
+    add_child($$, $3);
+  }
+|       term MOD factor {
+    $$ = create_expr(MOD_SYM);
+    add_child($$, $1);
+    add_child($$, $3);
+  }
+;
+
+factor: NUMBER {
+    $$ = create_const($1);
+  }
+|       L_PAREN exp R_PAREN {
+    $$ = $2;
+  }
+;
+
+un_op:  COMP {
+    $$ = COMP_SYM;
+  }
+|       MINUS {
+    $$ = NEG_SYM;
+  }
+;
+
+%%
+
+void yyerror(Node **root, const char *msg) {
+  printf("** Line %d: %s\n", yylineno, msg);
+}
+
diff --git a/src/modules/emitter/driver.scm b/src/modules/emitter/driver.scm
deleted file mode 100644
index 7ce8517..0000000
--- a/src/modules/emitter/driver.scm
+++ /dev/null
@@ -1,6 +0,0 @@
-(define-module (emitter driver)
-  #:export (assembly->string))
-
-
-(define (assembly->string n)
-  (eval n (resolve-module '(emitter traverse))))
diff --git a/src/modules/generator/driver.scm b/src/modules/generator/driver.scm
deleted file mode 100644
index 8b9ed30..0000000
--- a/src/modules/generator/driver.scm
+++ /dev/null
@@ -1,9 +0,0 @@
-(define-module (generator driver)
-  #:use-module (generator allocate)
-  #:export (tacky->assembly))
-
-
-(define (tacky->assembly n)
-  (eval (expansion->allocate
-         (eval n (resolve-module '(generator expansion))))
-        (resolve-module '(generator assembly))))
diff --git a/src/modules/lexer/driver.scm b/src/modules/lexer/driver.scm
deleted file mode 100644
index 92c8687..0000000
--- a/src/modules/lexer/driver.scm
+++ /dev/null
@@ -1,83 +0,0 @@
-(define-module (lexer driver)
-  #:export (read-tokens))
-
-(define (read-tokens)
-  "Returns a stream of tokens from the
-current input port."
-  (define (read-tokens-loop tokens-so-far)
-    (let ((token (read-token)))
-      (if token
-          (read-tokens-loop (cons token tokens-so-far))
-          (reverse tokens-so-far))))
-  (read-tokens-loop '()))
-
-(define (read-token)
-  (let ((chr (read-char)))
-    (cond
-     ((eof-object? chr) #f)
-     ((char-whitespace? chr)
-      (read-token))
-
-      ((eqv? chr #\() 'left-paren)
-      ((eqv? chr #\)) 'right-paren)
-      ((eqv? chr #\{) 'open-brace)
-      ((eqv? chr #\}) 'close-brace)
-      ((eqv? chr #\;) 'semi-colon)
-      ((eqv? chr #\~) 'not)
-      ((eqv? chr #\*) 'mul)
-      ((eqv? chr #\/) 'div)
-      ((eqv? chr #\%) 'mod)
-
-      ((eqv? chr #\+)
-       (if (take-double? chr)
-           '++
-           'add))
-
-      ((eqv? chr #\-)
-       (if (take-double? chr)
-           '--
-           'neg))
-
-      ((char-numeric? chr)
-       (read-constant chr))
-
-      ((char-alphabetic? chr)
-       (lookup-keyword (read-identifier chr)))
-
-      (#t (error "illegal lexical syntax")))))
-
-(define (read-constant chr)
-  (define (read-constant-helper chrs-so-far)
-    (let ((chr (peek-char)))
-      (cond ((and (not (eof-object? chr)) (char-numeric? chr))
-             (read-constant-helper (cons (read-char) chrs-so-far)))
-            ((and (not (eof-object? chr)) (char-alphabetic? chr))
-             (error "identifier starting with digit"))
-            (#t (reverse chrs-so-far)))))
-  (string->number (list->string (read-constant-helper (list chr)))))
-
-(define (read-identifier chr)
-  (define (read-identifier-helper chrs-so-far)
-    (let ((chr (peek-char)))
-      (cond ((and (not (eof-object? chr))
-              (or (char-alphabetic? chr)
-                  (char-numeric? chr)
-                  (eqv? chr #\_)))
-             (read-identifier-helper (cons (read-char) chrs-so-far)))
-            (#t (reverse chrs-so-far)))))
-  (list->string (read-identifier-helper (list chr))))
-
-(define (take-double? chr)
-  (if (eqv? chr (peek-char))
-      (read-char)
-      #f))
-
-(define (lookup-keyword id)
-  "Given identifier ID, converts it to a keyword
-if one is known."
-  (let ((found (assoc
-                id
-                '(("int" . int)
-                  ("void" . void)
-                  ("return" . return)))))
-    (if found (cdr found) id)))
diff --git a/src/modules/parser/driver.scm b/src/modules/parser/driver.scm
deleted file mode 100644
index e464a7a..0000000
--- a/src/modules/parser/driver.scm
+++ /dev/null
@@ -1,35 +0,0 @@
-(define-module (parser driver)
-  #:use-module (ice-9 match)
-  #:export (tokens->ast))
-
-
-(define (die)
-  (error "syntax error"))
-
-(define (tokens->ast tokens)
-  (match tokens
-    ((func ...)
-     (list 'prog (function func)))
-    (_ (die))))
-
-(define (function tokens)
-  (match tokens
-    (('int (? string? id) 'left-paren 'void 'right-paren 'open-brace stmt ... 'close-brace)
-     (list 'func id (statement stmt)))
-    (_ (die))))
-
-(define (statement tokens)
-  (match tokens
-    (('return expr ... 'semi-colon)
-     (list 'stmt (expression expr)))
-    (_ (die))))
-
-(define (expression tokens)
-  (match tokens
-    (((? integer? int))
-     (list 'const int))
-    (((or 'neg 'not) expr ...)
-     (list 'expr (car tokens) (expression expr)))
-    (('left-paren expr ... 'right-paren)
-     (expression expr))
-    (_ (die))))
diff --git a/src/modules/tacky/driver.scm b/src/modules/tacky/driver.scm
deleted file mode 100644
index 9696306..0000000
--- a/src/modules/tacky/driver.scm
+++ /dev/null
@@ -1,6 +0,0 @@
-(define-module (tacky driver)
-  #:export (ast->tacky))
-
-
-(define (ast->tacky n)
-  (eval n (resolve-module '(tacky traverse))))
diff --git a/src/ull b/src/ull
deleted file mode 100755
index f878c4e..0000000
--- a/src/ull
+++ /dev/null
@@ -1,107 +0,0 @@
-#!/run/current-system/profile/bin/guile \
--L ./src/modules -e main -s
-!#
-
-
-(use-modules (ice-9 getopt-long)
-             (ice-9 popen)
-             (ice-9 pretty-print)
-             (lexer driver)
-             (parser driver)
-             (tacky driver)
-             (generator driver)
-             (emitter driver))
-
-
-(define version "v0.1.1")
-
-
-(define (error message)
-  (display (string-concatenate `(,message "
-Usage:
-        ull [OPTIONS] file
-Options:
-        --version, -v: print version information
-        --debug,   -d: turn on verbose output
-        --lex,     -l: run the lexer, but stop before assembly generation
-        --parse,   -p: run the lexer and parser, but stop before assembly generation
-        --tacky,   -t: run the tacky generation stage, but stop before assembly generation
-        --codegen, -c: perform lexing, parsing, tacky, and assembly generation, but stop before code emission\n")))
-  (exit #f))
-
-(define (c-extension? file)
-  (let ((extension (string-drop file (- (string-length file) 2))))
-    (string=? extension ".c")))
-
-(define (process file parse? tack? generate? write? dest)
-  "Driver for lexing, parsing, and assembly generation."
-  (let* ((port (preprocess file))
-         (tokens (begin (set-current-input-port port)
-                        (read-tokens))))
-    (close-input-port port)
-    (when parse?
-      (let ((c-ast (tokens->ast tokens)))
-        (pretty-print c-ast)
-        (when tack?
-          (let ((tacky-ast (ast->tacky c-ast)))
-            (pretty-print tacky-ast)
-            (when generate?
-              (let ((assembly-ast (tacky->assembly tacky-ast)))
-                (pretty-print assembly-ast)
-                (when write?
-                  (let ((program (assembly->string assembly-ast))
-                        (a-file (string-append dest ".s")))
-                    (when (file-exists? a-file)
-                      (delete-file a-file))
-                    (let ((port (open-output-file a-file)))
-                      (display program port)
-                      (close-port port))
-                    (when (postprocess a-file dest)
-                      (display (string-concatenate `("Postprocess reported success (wrote " ,dest ").\n"))))
-                    ))))))))))
-
-(define (preprocess file)
-  "Returns an input port containing FILE processed with gcc."
-  (open-input-pipe (string-append "gcc -E -P " file)))
-
-(define (postprocess src dest)
-  "Assembles and links SRC, producing executable DEST.
-Returns #f on a failure, #t on a success."
-  (zero? (system (string-concatenate `("gcc " ,src " -o " ,dest)))))
-
-(define (main args)
-  "Entry point for ull. Handles user args and performs initial validity check."
-  (let* ((option-spec
-          '((version (single-char #\v) (value #f))
-            (debug   (single-char #\d) (value #f))
-            (lex     (single-char #\l) (value #f))
-            (parse   (single-char #\p) (value #f))
-            (tacky   (single-char #\t) (value #f))
-            (codegen (single-char #\c) (value #f))))
-         (options (getopt-long args option-spec))
-         (rest (option-ref options '() #f))
-         (file (if (null? rest) #f (car rest))))
-    (cond
-     ((option-ref options 'version #f)
-      (display (string-concatenate `("ull (" ,version ")\n"))))
-     ((not (equal? 1 (length rest))) (error "Wrong number of arguments."))
-     ((or (not file)
-          (not (access? file R_OK))
-          (not (equal? 'regular (stat:type (stat file))))
-          (not (c-extension? file))) (error "The file could not be read, or it is not a C source code file."))
-     (#t
-      (let ((parse? (not (option-ref options 'lex #f)))
-            (tack? (not (option-ref options 'parse #f)))
-            (generate? (not (option-ref options 'tacky #f)))
-            (write? (not (option-ref options 'codegen #f))))
-        (process file
-                 parse?
-                 tack?
-                 generate?
-                 write?
-                 (string-drop-right file 2)))))))
-
-
-;; Local Variables:
-;; mode: scheme
-;; End:
diff --git a/src/unit-tests/lexer/lexer.test.scm b/src/unit-tests/lexer/lexer.test.scm
deleted file mode 100644
index ce3520d..0000000
--- a/src/unit-tests/lexer/lexer.test.scm
+++ /dev/null
@@ -1,89 +0,0 @@
-;; -*- compile-command: "guile -L ./src/modules ./src/unit-tests/lexer/lexer.test.scm"; -*-
-(use-modules (srfi srfi-64)
-             (lexer lexer))
-
-(define (read-this str)
-  (set-current-input-port
-   (open-input-string str))
-  str)
-
-
-(test-begin "lexer-harness")
-
-
-(test-equal (read-this "")
-  '()
-  (read-tokens))
-
-(test-equal (read-this "(")
-  '(left-paren)
-  (read-tokens))
-
-(test-equal (read-this "((")
-  '(left-paren left-paren)
-  (read-tokens))
-
-(test-equal (read-this "( )")
-  '(left-paren right-paren)
-  (read-tokens))
-
-(test-equal (read-this "( {;} {((};})")
-  '(left-paren open-brace semi-colon close-brace open-brace left-paren left-paren close-brace semi-colon close-brace right-paren)
-  (read-tokens))
-
-(test-equal (read-this "1")
-  '(1)
-  (read-tokens))
-
-(test-equal (read-this "~+-")
-  '(complement add sub)
-  (read-tokens))
-
-(test-equal (read-this "---")
-  '(decrement sub)
-  (read-tokens))
-
-(test-equal (read-this "+--")
-  '(add decrement)
-  (read-tokens))
-
-(test-equal (read-this "0")
-  '(0)
-  (read-tokens))
-
-(test-equal (read-this "0011001")
-  '(11001)
-  (read-tokens))
-
-(test-equal (read-this "12 {34")
-  '(12 open-brace 34)
-  (read-tokens))
-
-(test-equal (read-this "34;")
-  '(34 semi-colon)
-  (read-tokens))
-
-(test-error (read-this "3.4")
-            (read-tokens))
-
-(test-equal (read-this "a")
-  '("a")
-  (read-tokens))
-
-(test-equal (read-this "a_2")
-  '("a_2")
-  (read-tokens))
-
-(test-error (read-this "1foo")
-            (read-tokens))
-
-(test-equal (read-this "void")
-  '(void)
-  (read-tokens))
-
-(test-equal (read-this "int main(void) {return 2;}")
-  '(int "main" left-paren void right-paren open-brace return 2 semi-colon close-brace)
-  (read-tokens))
-
-
-(test-end "lexer-harness")
diff --git a/src/unit-tests/parser/parser.test.scm b/src/unit-tests/parser/parser.test.scm
deleted file mode 100644
index 32e1e6c..0000000
--- a/src/unit-tests/parser/parser.test.scm
+++ /dev/null
@@ -1,30 +0,0 @@
-;; -*- compile-command: "guile -L ./src/modules ./src/unit-tests/parser/parser.test.scm"; -*-
-(use-modules (srfi srfi-64)
-             (parser parser))
-
-
-(test-begin "parser-harness")
-
-
-(test-equal "trivial function main 2"
-  '(program (function (identifier "main") (return (constant 2))))
-  (p-program '(int "main" left-paren void right-paren open-brace return 2 semi-colon close-brace)))
-
-(test-equal "trivial function foo 4"
-  '(program (function (identifier "foo") (return (constant 4))))
-  (p-program '(int "foo" left-paren void right-paren open-brace return 4 semi-colon close-brace)))
-
-(test-error "trivial function bad double return"
-            (p-program '(int "foo" left-paren void right-paren open-brace return return 4 semi-colon close-brace)))
-
-(test-error "trivial function bad parens"
-            (p-program '(int "foo" right-paren void left-paren open-brace return 4 semi-colon close-brace)))
-
-(test-error "trivial function bad int parameter"
-            (p-program '(int "foo" left-paren int right-paren open-brace return 4 semi-colon close-brace)))
-
-(test-error "trivial function incomplete function"
-            (p-program '(int "foo" left-paren void right-paren open-brace return)))
-
-
-(test-end "parser-harness")
diff --git a/src/unit-tests/utils/t-factory.test.scm b/src/unit-tests/utils/t-factory.test.scm
deleted file mode 100644
index d780c17..0000000
--- a/src/unit-tests/utils/t-factory.test.scm
+++ /dev/null
@@ -1,19 +0,0 @@
-;; -*- compile-command: "guile -L ./src/modules ./src/unit-tests/utils/t-factory.test.scm"; -*-
-(use-modules (srfi srfi-64)
-             (ast assembly-tree)
-             (utils t-factory))
-
-
-(test-begin "t-factory-harness")
-
-
-(test-equal "make first temporary"
-  't.101
-  (temporary-name (make-t)))
-
-(test-equal "make second temporary"
-  't.102
-  (temporary-name (make-t)))
-
-
-(test-end "t-factory-harness")
diff --git a/ull b/ull
new file mode 100755
index 0000000..004d7e5
--- /dev/null
+++ b/ull
@@ -0,0 +1,127 @@
+#!/run/current-system/profile/bin/guile \
+-L ./src -e main -s
+!#
+
+(use-modules (ice-9 getopt-long)
+             (ice-9 popen)
+             (ice-9 pretty-print)
+	     (frontend driver)
+	     (backend tacky driver)
+             (backend generator driver)
+             (backend emitter driver))
+
+(define version "v0.1.1")
+
+(define (error message)
+  (display (string-concatenate `(,message "
+Usage:
+        ull [OPTIONS] file
+Options:
+        --version, -v: print version information
+        --debug,   -d: turn on verbose output
+        --lex,     -l: run the lexer, but stop before assembly generation
+        --parse,   -p: run the lexer and parser, but stop before assembly generation
+        --tacky,   -t: run the tacky generation stage, but stop before assembly generation
+        --codegen, -c: perform lexing, parsing, tacky, and assembly generation, but stop before code emission\n")))
+  (exit #f))
+
+(define (c-extension? file)
+  (let ((extension (string-drop file (- (string-length file) 2))))
+    (string=? extension ".c")))
+
+(define (cleanup-file file)
+  (when (file-exists? file)
+    (delete-file file)))
+
+(define (preprocess src dst)
+  "Preprocesses SRC with gcc and write it to DST."
+  (system (string-concatenate `("gcc -E -P " ,src " -o " ,dst)))
+  (display (string-concatenate `("Preprocess reported success (wrote " ,dst ").\n"))))
+
+(define (frontend file parse?)
+  "Wrapper for lexing and parsing. Returns an AST representing FILE."
+  (let ((c-ast (file->ast file parse?)))
+    (or c-ast
+	(begin
+	  (display (format #f "~a reported failure!\n" (or (and parse? "Parser") "Lexer")))
+	  (cleanup-file file)
+	  (exit #f)))))
+
+(define (backend c-ast tack? generate? write?)
+  "Driver for tacky and assembly generation, code emission. Returns an assembly program representing C-AST."
+  (pretty-print c-ast)
+  (display "===^file->ast^===\n")
+  (when tack?
+    (let ((tacky-ast (ast->tacky c-ast)))
+      (pretty-print tacky-ast)
+      (display "===^ast->tacky^===\n")
+      (when generate?
+	(let ((assembly-ast (tacky->assembly tacky-ast)))
+	  (pretty-print assembly-ast)
+	  (display "===^tacky->assembly^===\n")
+	  (when write?
+	    (assembly->string assembly-ast)))))))
+
+(define (postprocess src dest)
+  "Assembles and links SRC, producing executable DEST.
+Returns #f on a failure, #t on a success."
+  (zero? (system (string-concatenate `("gcc " ,src " -o " ,dest)))))
+
+(define (write str dst)
+  "Writes STR to the file at DST."
+  (cleanup-file dst)
+  (let ((port (open-output-file dst)))
+    (display str port)
+    (close-port port))
+  (display (string-concatenate `("Assembly generation reported success (wrote " ,dst ").\n"))))
+
+(define (main args)
+  "Entry point for ull. Handles user args and performs initial validity check."
+  (let* ((option-spec
+          '((version (single-char #\v) (value #f))
+            (debug   (single-char #\d) (value #f))
+            (lex     (single-char #\l) (value #f))
+            (parse   (single-char #\p) (value #f))
+            (tacky   (single-char #\t) (value #f))
+            (codegen (single-char #\c) (value #f))))
+	 (options (getopt-long args option-spec))
+	 (rest (option-ref options '() #f))
+	 (file-name (if (null? rest) #f (car rest))) )
+    (cond
+     ;; display version number
+     ((option-ref options 'version #f)
+      (display (string-concatenate `("ull (" ,version ")\n"))))
+
+     ;; complain about usage
+     ((not (equal? 1 (length rest))) (error "Wrong number of arguments."))
+     ((or (not file-name)
+          (not (access? file-name R_OK))
+          (not (equal? 'regular (stat:type (stat file-name))))
+          (not (c-extension? file-name))) (error "The file could not be read, or it is not a C source code file."))
+
+     (#t
+      (let ((parse? (not (option-ref options 'lex #f)))
+            (tack? (not (option-ref options 'parse #f)))
+            (generate? (not (option-ref options 'tacky #f)))
+            (write? (not (option-ref options 'codegen #f)))
+	    (preprocessed-file-name
+	     (string-concatenate `(,(string-drop-right file-name 1) "i")))
+	    (assembly-file-name
+	     (string-concatenate `(,(string-drop-right file-name 1) "s")))
+	    (executable-file-name
+	     (string-drop-right file-name 2)))
+
+	;; preprocess the file
+	(preprocess file-name preprocessed-file-name)
+	;; call the frontend
+	(let ((c-ast (frontend preprocessed-file-name parse?)))
+	  (cleanup-file preprocessed-file-name)
+	  (if (and parse? c-ast)
+	      ;; call the backend
+	      (begin (display "Parser reported success\n")
+		     (let ((program (backend c-ast tack? generate? write?)))
+		       (when write?
+			 (write program assembly-file-name)
+			 ;; call postprocessing
+			 (postprocess assembly-file-name executable-file-name))))
+	      (display "Tokenizer successful. "))))))))