diff --git a/CMakeLists.txt b/CMakeLists.txt
index 11bbe0f..98dd6db 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -22,6 +22,7 @@ target_sources(vm_lib
   src/opcode.cpp
   src/fio.cpp
   src/stdlib.cpp
+  src/lineedit.cpp
 
   PUBLIC
   FILE_SET HEADERS
@@ -41,6 +42,7 @@ target_sources(vm_lib
   src/opcode.hpp
   src/fio.hpp
   src/stdlib.hpp
+  src/lineedit.hpp
 )
 
 add_executable(vli src/vli.cpp)
diff --git a/src/common.hpp b/src/common.hpp
index 95af76b..c1ccaae 100644
--- a/src/common.hpp
+++ b/src/common.hpp
@@ -204,7 +204,7 @@ class Array : public Object {
     return Array(TRY(MkGcRoot(pod)));
   }
 
-  Result<Array> slice(uint64_t start, uint64_t end) {
+  Result<Array> slice(uint64_t start, uint64_t end) const {
     if (start > end) return ERROR(IndexOutOfRange);
     uint64_t res_size = end - start;
     auto pod = TRY(arena_alloc<PodArray>(res_size * sizeof(PodObject*)));
@@ -315,7 +315,7 @@ class ByteArray : public Object {
     return ByteArray(TRY(MkGcRoot(pod)));
   }
 
-  Result<ByteArray> slice(uint64_t start, uint64_t end) {
+  Result<ByteArray> slice(uint64_t start, uint64_t end) const {
     if (start > end) return ERROR(IndexOutOfRange);
     uint64_t res_size = end - start;
     auto pod = TRY(arena_alloc<PodByteArray>(res_size * sizeof(char)));
@@ -510,7 +510,7 @@ class String : public Object {
     return String(TRY(MkGcRoot(pod)));
   }
 
-  Result<String> slice(uint64_t start, uint64_t end) {
+  Result<String> slice(uint64_t start, uint64_t end) const {
     if (start > end) return ERROR(IndexOutOfRange);
     uint64_t res_size = end - start;
     auto pod = TRY(arena_alloc<PodString>(res_size * sizeof(char32_t)));
diff --git a/src/error.hpp b/src/error.hpp
index dced58c..e3e68e1 100644
--- a/src/error.hpp
+++ b/src/error.hpp
@@ -16,6 +16,7 @@ enum class ErrorCode {
   CompilationError,
   ArgumentCountMismatch,
   IOError,
+  Interrupt,
 };
 
 void seterr(const char* err);
diff --git a/src/lineedit.cpp b/src/lineedit.cpp
new file mode 100644
index 0000000..efb552a
--- /dev/null
+++ b/src/lineedit.cpp
@@ -0,0 +1,394 @@
+#include "lineedit.hpp"
+
+#include <sys/ioctl.h>
+#include <termios.h>
+#include <unistd.h>
+
+#include <cstdlib>
+
+#include "common.hpp"
+#include "fio.hpp"
+
+enum class TtyEscape {
+  Null = 0,
+  CtrlA = 1,
+  CtrlB = 2,
+  CtrlC = 3,
+  CtrlD = 4,
+  CtrlE = 5,
+  CtrlF = 6,
+  CtrlH = 8,
+  Tab = 9,
+  CtrlK = 11,
+  CtrlL = 12,
+  Enter = 13,
+  CtrlN = 14,
+  CtrlP = 16,
+  CtrlT = 20,
+  CtrlU = 21,
+  CtrlW = 23,
+  Esc = 27,
+  Backspace = 127,
+};
+
+static struct termios orig_termios;
+static int rawmode = 0;
+
+template <uint64_t buflen>
+class CharBuf {
+ public:
+  CharBuf() : _len(0) {}
+
+  Result<void> append(char c) {
+    if (_len + 1 >= buflen) return ERROR(IndexOutOfRange);
+    _buf[_len] = c;
+    _len++;
+
+    return Result<void>();
+  }
+  Result<void> append(const char* s) {
+    uint64_t slen = strlen(s);
+    if (_len + slen >= buflen) return ERROR(IndexOutOfRange);
+
+    memcpy(_buf + _len, s, slen);
+    _len += slen;
+
+    return Result<void>();
+  }
+
+  uint64_t len() { return _len; }
+  const char* buf() { return _buf; }
+
+ private:
+  char _buf[buflen];
+  uint64_t _len;
+};
+
+Result<void> enableRawMode(int fd) {
+  struct termios raw;
+
+  if (!isatty(STDIN_FILENO)) return ERROR(IOError);
+  if (tcgetattr(fd, &orig_termios) == -1) return ERROR(IOError);
+
+  raw = orig_termios; /* modify the original mode */
+  /* input modes: no break, no CR to NL, no parity check, no strip char,
+   * no start/stop output control. */
+  raw.c_iflag &= ~(BRKINT | ICRNL | INPCK | ISTRIP | IXON);
+  /* output modes - disable post processing */
+  raw.c_oflag &= ~(OPOST);
+  /* control modes - set 8 bit chars */
+  raw.c_cflag |= (CS8);
+  /* local modes - choing off, canonical off, no extended functions,
+   * no signal chars (^Z,^C) */
+  raw.c_lflag &= ~(ECHO | ICANON | IEXTEN | ISIG);
+  /* control chars - set return condition: min number of bytes and timer.
+   * We want read to return every single byte, without timeout. */
+  raw.c_cc[VMIN] = 1;
+  raw.c_cc[VTIME] = 0; /* 1 byte, no timer */
+
+  /* put terminal in raw mode after flushing */
+  if (tcsetattr(fd, TCSAFLUSH, &raw) < 0) return ERROR(IOError);
+  rawmode = 1;
+
+  return Result<void>();
+}
+
+void disableRawMode(int fd) {
+  /* Don't even check the return value as it's too late. */
+  if (rawmode && tcsetattr(fd, TCSAFLUSH, &orig_termios) != -1) rawmode = 0;
+}
+
+class RawModeGuard {
+ public:
+  RawModeGuard() : fd(-1) {}
+  RawModeGuard(int fd) : fd(fd) {}
+  RawModeGuard(RawModeGuard&& rhs) {
+    fd = rhs.fd;
+    rhs.fd = -1;
+  }
+  RawModeGuard(const RawModeGuard&) = delete;
+  ~RawModeGuard() {
+    if (fd >= 0) disableRawMode(fd);
+  }
+
+  static Result<RawModeGuard> create(int fd) {
+    auto res = RawModeGuard(fd);
+    TRY(enableRawMode(fd));
+
+    return res;
+  }
+
+ private:
+  int fd;
+};
+
+RawModeGuard raw_mode_guard(STDIN_FILENO);
+
+struct RefreshState {
+  uint64_t num_rows = 0;
+  uint64_t cursor_pos = 0;
+};
+
+class LineEdit {
+ public:
+  LineEdit()
+      : prompt(""),
+        cursor_pos(0),
+        line_len(0),
+        term_width(0),
+        input_fd(STDIN_FILENO),
+        output_fd(STDOUT_FILENO) {
+    buf[0] = 0;
+  }
+
+  LineEdit(const char* prompt, int input_fd = STDIN_FILENO,
+           int output_fd = STDOUT_FILENO)
+      : prompt(prompt),
+        cursor_pos(0),
+        line_len(0),
+        term_width(0),
+        input_fd(input_fd),
+        output_fd(output_fd) {
+    buf[0] = 0;
+  }
+
+  LineEdit(const LineEdit& rhs) {
+    prompt = rhs.prompt;
+    memcpy(buf, rhs.buf, buflen);
+    cursor_pos = rhs.cursor_pos;
+    line_len = rhs.line_len;
+    term_width = rhs.term_width;
+    input_fd = rhs.input_fd;
+    output_fd = rhs.output_fd;
+    last_refresh = rhs.last_refresh;
+  }
+
+  static Result<LineEdit> create(const char* prompt) {
+    LineEdit res(prompt);
+
+    TRY(res.init());
+
+    return res;
+  }
+
+  Result<uint64_t> get_term_width() {
+    winsize ws;
+
+    if (ioctl(1, TIOCGWINSZ, &ws) == -1 || ws.ws_col == 0) {
+      /* ioctl() failed. Try to query the terminal itself. */
+      int start, cols;
+
+      /* Get the initial position so we can restore it later. */
+      start = TRY(get_cursor_column());
+
+      /* Go to right margin and get position. */
+      if (write(output_fd, "\x1b[999C", 6) != 6) return ERROR(IOError);
+      cols = TRY(get_cursor_column());
+
+      /* Restore position. */
+      if (cols > start) {
+        char seq[32];
+        snprintf(seq, 32, "\x1b[%dD", cols - start);
+        if (write(output_fd, seq, strlen(seq)) == -1) {
+          return ERROR(IOError);
+        }
+      }
+      return cols;
+    } else {
+      return ws.ws_col;
+    }
+  }
+
+  Result<uint64_t> get_cursor_column() {
+    char buf[32];
+    int cols, rows;
+    unsigned int i = 0;
+
+    /* Report cursor location */
+    if (write(output_fd, "\x1b[6n", 4) != 4) return -1;
+
+    /* Read the response: ESC [ rows ; cols R */
+    while (i < sizeof(buf) - 1) {
+      if (read(input_fd, buf + i, 1) != 1) break;
+      if (buf[i] == 'R') break;
+      i++;
+    }
+    buf[i] = '\0';
+
+    /* Parse it. */
+    if (buf[0] != (char)TtyEscape::Esc || buf[1] != '[') return -1;
+    if (sscanf(buf + 2, "%d;%d", &rows, &cols) != 2) return -1;
+    return cols;
+  }
+
+  Result<String> read_one() {
+    {
+      auto guard = TRY(RawModeGuard::create(input_fd));
+      if (write(output_fd, prompt, strlen(prompt)) == -1) return ERROR(IOError);
+
+      while (TRY(step()));
+    }
+    printf("\n");
+
+    auto res = TRY(String::create(buf));
+
+    return res;
+  }
+
+  Result<bool> step() {
+    char c;
+    int nread;
+    char seq[3];
+
+    nread = read(input_fd, &c, 1);
+    if (nread <= 0) return false;
+
+    switch (TtyEscape(c)) {
+      case TtyEscape::Enter:
+        return false;
+        break;
+      case TtyEscape::Backspace:
+        TRY(handle_backspace());
+        break;
+      case TtyEscape::CtrlC:
+        return ERROR(Interrupt);
+      default:
+        TRY(handle_regular_char(c));
+        break;
+    }
+    return true;
+  }
+
+  Result<void> handle_regular_char(char c) {
+    if (line_len >= buflen) return Result<void>();
+
+    if (line_len == cursor_pos) {
+      buf[cursor_pos] = c;
+      cursor_pos++;
+      line_len++;
+      buf[cursor_pos] = 0;
+
+      // if (strlen(prompt) + line_len < term_width) {
+      // if (write(output_fd, &c, 1) == -1) return ERROR(IOError);
+      // } else {
+      TRY(refresh_line());
+      //}
+    } else {
+      memmove(buf + cursor_pos + 1, buf + cursor_pos, line_len - cursor_pos);
+      buf[cursor_pos] = c;
+      cursor_pos++;
+      line_len++;
+      buf[line_len] = 0;
+
+      TRY(refresh_line());
+    }
+    return Result<void>();
+  }
+
+  Result<void> handle_backspace() {
+    if (cursor_pos == 0) return Result<void>();
+    memmove(buf + cursor_pos - 1, buf + cursor_pos, line_len - cursor_pos);
+    cursor_pos--;
+    line_len--;
+    buf[line_len] = 0;
+
+    TRY(refresh_line());
+
+    return Result<void>();
+  }
+
+  Result<void> refresh_line() {
+    char seq[64];
+    CharBuf<1024> cbuf;
+
+    RefreshState new_refresh = last_refresh;
+
+    uint64_t prompt_len = strlen(prompt);
+
+    uint64_t num_rows = (prompt_len + line_len + term_width - 1) / term_width;
+    uint64_t cursor_row =
+        (prompt_len + last_refresh.cursor_pos + term_width) / term_width;
+    uint64_t new_relative_row = 0;
+
+    new_refresh.num_rows = num_rows;
+    new_refresh.cursor_pos = cursor_pos;
+
+    // Go to last row
+    if (last_refresh.num_rows - cursor_row > 0) {
+      snprintf(seq, 64, "\x1b[%luB", last_refresh.num_rows - cursor_row);
+      TRY(cbuf.append(seq));
+    }
+
+    // Clear all rows one by one (except the first one)
+    if (last_refresh.num_rows > 1) {
+      for (uint64_t i = 0; i < last_refresh.num_rows - 1; i++) {
+        snprintf(seq, 64, "\r\x1b[0K\x1b[1A");
+        TRY(cbuf.append(seq));
+      }
+    }
+
+    // Clear the first line
+    snprintf(seq, 64, "\r\x1b[0K");
+    TRY(cbuf.append(seq));
+
+    // Write the prompt
+    cbuf.append(prompt);
+
+    // Write the buffer content
+    cbuf.append(buf);
+
+    if (cursor_pos > 0 && cursor_pos == line_len &&
+        (cursor_pos + prompt_len) % term_width == 0) {
+      cbuf.append("\n");
+      cbuf.append("\r");
+      num_rows++;
+      if (num_rows > last_refresh.num_rows) new_refresh.num_rows = num_rows;
+    }
+
+    cursor_row = (prompt_len + cursor_pos + term_width) / term_width;
+
+    // Go up to the cursor row
+    if (num_rows - cursor_row > 0) {
+      snprintf(seq, 64, "\x1b[%luA", num_rows - cursor_row);
+      TRY(cbuf.append(seq));
+    }
+
+    // Go to the cursor column
+    uint64_t col = (prompt_len + cursor_pos) % term_width;
+    if (col > 0) {
+      snprintf(seq, 64, "\r\x1b[%luC", col);
+      TRY(cbuf.append(seq));
+    } else {
+      TRY(cbuf.append("\r"));
+    }
+
+    if (write(output_fd, cbuf.buf(), cbuf.len()) == -1) return ERROR(IOError);
+
+    last_refresh = new_refresh;
+    return Result<void>();
+  }
+
+ private:
+  Result<void> init() {
+    term_width = TRY(get_term_width());
+
+    if (!stdin_isatty()) return ERROR(IOError);
+    return Result<void>();
+  }
+
+  const char* prompt;
+  static const uint64_t buflen = 1024;
+  char buf[buflen + 1];
+  uint64_t cursor_pos;
+  uint64_t line_len;
+  uint64_t term_width;
+  int input_fd;
+  int output_fd;
+  RefreshState last_refresh;
+};
+
+Result<String> read_line(const char* prompt) {
+  auto rl = TRY(LineEdit::create(prompt));
+
+  return rl.read_one();
+}
diff --git a/src/lineedit.hpp b/src/lineedit.hpp
new file mode 100644
index 0000000..75583a2
--- /dev/null
+++ b/src/lineedit.hpp
@@ -0,0 +1,6 @@
+#pragma once
+
+#include "result.hpp"
+
+class String;
+Result<String> read_line(const char* prompt);
diff --git a/src/reader.cpp b/src/reader.cpp
index 492d0f9..410ae60 100644
--- a/src/reader.cpp
+++ b/src/reader.cpp
@@ -409,12 +409,12 @@ bool Reader::forward_exponent() {
   return forward_decimal_number();
 }
 
-Result<Value> read_one(Value& value) {
+Result<Value> read_one(const Value& value) {
   if (!value.is<String>()) return ERROR(TypeMismatch);
   auto r = Reader(*value.to<String>());
   return r.read_one();
 }
-Result<Value> read_one(String& value) {
+Result<Value> read_one(const String& value) {
   auto r = Reader(value);
   return r.read_one();
 }
@@ -424,7 +424,7 @@ Result<Value> read_one(const char* value) {
   return r.read_one();
 }
 
-Result<Value> read_multiple(String& value) {
+Result<Value> read_multiple(const String& value) {
   auto r = Reader(value);
   return r.read_multiple();
 }
diff --git a/src/reader.hpp b/src/reader.hpp
index eeba149..da0954c 100644
--- a/src/reader.hpp
+++ b/src/reader.hpp
@@ -5,7 +5,7 @@
 
 class Reader {
  public:
-  Reader(String& str) : _str(str) {}
+  Reader(const String& str) : _str(str) {}
 
   Result<Value> read_one();
   Result<Value> read_multiple();
@@ -38,12 +38,12 @@ class Reader {
   bool match(const char* str);
   bool match(char c);
 
-  String& _str;
+  const String& _str;
   SourcePosition position_{1, 1, 0};
 };
 
-Result<Value> read_one(Value& value);
-Result<Value> read_one(String& value);
+Result<Value> read_one(const Value& value);
+Result<Value> read_one(const String& value);
 Result<Value> read_one(const char* value);
 
-Result<Value> read_multiple(String& value);
+Result<Value> read_multiple(const String& value);
diff --git a/src/vli.cpp b/src/vli.cpp
index adacd1d..ed516c8 100644
--- a/src/vli.cpp
+++ b/src/vli.cpp
@@ -5,44 +5,51 @@
 #include "compiler.hpp"
 #include "die.hpp"
 #include "fio.hpp"
+#include "lineedit.hpp"
 #include "reader.hpp"
 #include "vm.hpp"
 #include "writer.hpp"
 
 StaticArena<64 * 1024 * 1024> arena;
 
-Result<void> run(int argc, const char* argv[]) {
-  String src = DIEX(String::create(""));
-  if (argc == 1) {
-    if (stdin_isatty()) {
-      die("Code expected at stdin, not a tty.\n");
-    }
-    src = TRY(read_stdin());
-  } else {
-    src = TRY(read_file(argv[1]));
-  }
-
-  // auto code_str = TRY(String::create("(* (+ 1 2 3) (/ 4 2))"));
-  // auto code_str =
-  // TRY(String::create("((lambda (f y) (f y)) (lambda (x) (* x x)) 2)"));
-
+Result<Value> run_string(const String& src) {
   auto parsed = TRY(read_multiple(src));
-  // auto code_str_written = TRY(write_multiple(parsed));
 
   TRY(arena_gc());
 
-  // TRY(debug_print(code_str_written));
-
   auto compiled = TRY(compile(parsed));
   Module& mod = *compiled.to<Module>();
 
-  // TRY(debug_print(TRY(mod.globals())));
-
   auto vm = TRY(VM::create());
 
   auto res = TRY(vm.run(mod));
 
-  // TRY(debug_print(res));
+  return res;
+}
+
+Result<void> run_repl() {
+  while (true) {
+    auto src = TRY(read_line("vli> "));
+    debug_print(src);
+    auto res = TRY(run_string(src));
+    debug_print(res);
+  }
+
+  return Result<void>();
+}
+
+Result<void> run(int argc, const char* argv[]) {
+  String src = DIEX(String::create(""));
+  if (argc == 1) {
+    if (stdin_isatty()) {
+      return run_repl();
+    }
+    src = TRY(read_stdin());
+    TRY(run_string(src));
+  } else {
+    src = TRY(read_file(argv[1]));
+    TRY(run_string(src));
+  }
 
   return Result<void>();
 }