finish project eight backend compiler

- modify existing implementation to use variant types for a more
  accurate representation of the vm byte code
- switch to fold from recursion for the main translate function
- use separate modules for translating different vm commands
- move static arithmetic command translations to a map

compiler/backend/bin/compiler.ml

@@ -0,0 +1,61 @@
+open Core
+open Backend
+
+let read_file file =
+  let lines = In_channel.read_lines file in
+  let not_empty str = not (String.is_empty str) in
+  List.filter lines ~f:not_empty
+
+
+let is_vm_file f =
+  match Filename.split_extension f with
+  | _, Some ext -> String.equal ext "vm"
+  | _, None -> false
+
+
+let generate_hack f =
+  let bytecode = read_file f in
+  let class_name = Filename.basename f in
+  Translate.translate class_name bytecode
+
+
+let write_hack path hack =
+  let output_channel = Out_channel.create path in
+  Out_channel.output_lines output_channel hack;
+  Out_channel.close output_channel
+
+
+let compile path =
+  let ext = ".asm" in
+  let absolute_path x = path ^ x in
+  let open Filename_base in
+    if Sys_unix.is_directory_exn path then
+      let output_file = concat path (basename path ^ ext) in
+      let hack =
+        path
+        |> Sys_unix.ls_dir
+        |> List.map ~f:absolute_path
+        |> List.filter ~f:is_vm_file
+        |> List.concat_map ~f:generate_hack
+        |> List.append Translate.bootstrap
+      in
+      write_hack output_file hack
+    else if Sys_unix.is_file_exn path && is_vm_file path then
+      let output_file = chop_extension path ^ ext in
+      let hack = generate_hack path in
+      write_hack output_file hack
+    else
+      failwith "Incompatible!"
+
+
+let command =
+  let summary = "Translate Jack byte code to Hack assembly" in
+  let readme () = "Compiler (backend) for project 7/8 of Nand2Tetris" in
+  let param_spec = Command.Param.(anon ("path" %: string)) in
+  let param_handler path () = compile path in
+  Command.basic
+    ~summary:summary ~readme:readme
+    (Command.Param.map param_spec ~f:param_handler)
+
+
+let () = Command_unix.run command

compiler/backend/bin/compiler.mli

@@ -0,0 +1 @@
+val command : Command.t

compiler/backend/bin/dune

@@ -0,0 +1,5 @@
+(executable
+ (modes byte exe)
+ (name compiler)
+ (public_name compiler)
+ (libraries backend core core_unix.command_unix))