Type Debugger for OCaml

September 24, 2016

News

(2015 May 21) OCaml 4.02.1 supported!
(2014 Oct 20) Initial release for OCaml 4.01.0.

What is the Type Debugger for OCaml?

Have you ever been confused by the type error messages produced by OCaml? If you have, you might have wondered why the OCaml compiler does not produce better error messages. The reason is: it can't. During type inference, the OCaml compiler finds a type conflict between two subexpressions: one requires one type and the other provides a different type. However, there is no way for the OCaml compiler to know which of the two types is the right type for the programmer. Thus, the OCaml compiler arbitrarily chooses one of them and reports it as a type error.

The Type Debugger in this page improves on this situation by asking questions to the programmer. From the answers to the questions, it understands the programmer's intention and leads the programmer to the real source of the type error. This idea of asking questions to obtain programmer's intention is due to Shapiro's algorithmic program debugging and is used by Chitil to design a type debugger for a small subset of Haskell. We extend this approach to support full OCaml by reusing the type inferencer in the OCaml compiler. The Type Debugger has been used successfully in the functional programming course in Ochanomizu University.

Environments

We are developing the Type Debugger using the following environments:

MacOSX 10.5 and later (64bit, Intel)
OCaml 4.01.0 and 4.02.1
Cocoa Emacs + tuareg mode, Vim
OCamlMakefile

Installation via OPAM

Install OPAM (e.g., via macports).
Make a new directory ~/.opam/compilers/4.02.1/4.02.1+type-debugger and place the two files 4.02.1+type-debugger.comp and 4.02.1+type-debugger.descr there.
Add the following line to the file ~/.opam/repo/compiler-index:
```
4.02.1+type-debugger default /4.02.1/4.02.1+type-debugger
```

Execute:

opam switch 4.02.1+type-debugger
opam install tuareg
opam install ocaml-makefile

For OCaml 4.01.0, use the files in 4.01.0 directory.

Download/Compile

The patch is placed under download directory. To compile, see 4.02.1+type-debugger.comp.

Emacs interface

Install the tuareg mode. (Do M-x byte-compile-file ~/.opam/4.XX.X+type-debugger/share/tuareg/tuareg.el if tuareg mode produces an error.)
Download the tuareg-ochadai.el of the elisp interface file and install it, e.g., under ~/.emacs.d/elisp/.
Edit the first three variables of tuareg-ochadai.el. If your default ocaml is the type debugger, you don't have to edit.
Insert a command to load tuareg-ochadai.el at an appropriate place (e.g., .emacs.d/init.el), such as:
```
(load "/path/to/tuareg-ochadai.el")
```

The tuareg-ochadai.el file supports:

Highlight Error: Highlight the last error (or warning) message in the *caml-toplevel* buffer.
Evaluate Buffer: Similar to tuareg-evaluate-buffer, but it first saves the current buffer, kill the OCaml toplevel, and load the file freshly.
Compile Program: Execute make in the current directory.
Answer Yes/No/Quit: Send the answer to the Type Debugger.

See the new tuareg menu for key combinations.

Vim interface

Install vimproc.
Download autoload.vim, rename it to typedebugger.vim, edit lines 20 and 25 appropriately (if the default ocaml binary is not the type debugger), and place it under .vim/autoload/ in your home directory.
Download plugin.vim, rename it to typedebugger.vim, edit line 11 appropriately (if the default ocaml binary is not the type debugger), and place it under .vim/plugin/ in your home directory.

To start the type debugger:

Type :TypeDebuggerStart to launch the type debugger.
Type :TypeDebuggerCompile to compile using make with the type debugger.

To debug:

To answer Yes: type (backslash followed by y) or :TypeDebuggerSendYes.
To answer No: type (backslash followed by n) or :TypeDebuggerSendNo.
To answer Quit: type (backslash followed by q) or :TypeDebuggerSendQuit.
To answer Exit: type (backslash followed by e) or :TypeDebuggerExit.

Papers and talks

Asai, K. ``Principle and Practice of OCaml Type Debugger,'' presented at the ACM SIGPLAN Programming Languages Mentoring Workshop at ICFP (September 2016). abstract, slides, YouTube
Ishii, Y., and K. Asai ``Report on a User Test and Extension of a Type Debugger for Novice Programmers,'' 3rd International Workshop on Trends in Functional Programming in Education (June 2014). Post conference version in Electronic Proceedings in Theoretical Computer Science 170, pp. 1-18 (December 2014).
Tsushima, K. ``Practicable type debugging for functional languages,'' PhD thesis, Department of Information Science, Ochanomizu University (September 2013). PDF
Tsushima, K., and K. Asai ``An Embedded Type Debugger,'' The 24th Symposium on Implementation and Application of Functional Languages, Oxford (August 2012). abstract. Post conference version in LNCS 8241, pp. 190-206 (2013). Received Peter Landin Prize.

Members

This document was translated from L^AT_EX by H^EV^EA.