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Formatting and Wrapping Text

Packing and Structural “hov” Boxes

the vertical or horizontal packing box (as obtained by the open_hovbox procedure): break hints are used to cut the line when there is no more room on the line; no new line occurs if there is

Formatting and Wrapping Text

Differences Between a Packing and a Structural “hov” Box

If we replace the packing boxes by structural boxes (open_box), each break hint that precedes a closing parenthesis can show the boxes structure, if it leads to a new line; hence [(---[(----[(

Formatting and Wrapping Text

Practical Advice

Boxes must be opened and closed consistently ( open_* and close_box must be nested like parentheses). Never hesitate to open a box. Output many break hints, otherwise the pretty-printer is in a

Formatting and Wrapping Text

Printing to stdout : Using printf

For instance “ @[ ” open a box ( open_box 0 ). You may precise the type as an extra argument. For instance @[<hov n> is equivalent to open_hovbox n . “ @] ” close a box (

Formatting and Wrapping Text

A Concrete Example

<!-- $MDX skip --> In Caml Light, replace the first line by: I use the format library to print the lambda-terms: First, I give the abstract syntax of lambda-terms: Thus the problem is to pr

Formatting and Wrapping Text

Most General Pretty-Printing: Using fprintf

Given those general printing routines, procedures to print to stdout or stderr is just a matter of partial application: Using fprintf , the lambda-terms printing routines can be written as

Command-line Arguments

Command-line Arguments

Sys.argv

<!-- $MDX dir=examples --> Note that ocaml launched a subprocess that actually runs the program where argv is args.ml arg1 arg2 arg3 . You can also compile your program using ocamlopt -o ar

Command-line Arguments

Using the Arg Module

The Arg module has many more actions than just Set and Set_string , and some lower-level function for parsing more complicated command lines. Here is the whole program: <!-- $MDX dir=exam

Command-line Arguments

Other Tools for Parsing Command-Line Options

Cmdliner is a modern interface for command line processing, which also generates UNIX man pages automatically. Clap is an imperative command line parser. Minicli has good support for rej

File Manipulation

File Manipulation

Buffered Channels

channels that write to a file: type out_channel channels that read from a file: type in_channel The normal way of opening a file in OCaml returns a channel . There are two kinds of channels:

File Manipulation

Writing

Standard out_channel s: stdout , stderr Commonly used functions: open_out , open_out_bin , flush , close_out , close_out_noerr Open the file to obtain an out_channel Write to the channel

File Manipulation

Reading

Standard in_channel : stdin Commonly used functions: open_in , open_in_bin , close_in , close_in_noerr Open the file to obtain an in_channel Read characters from the channel. Reading consum

File Manipulation

Seeking

Whenever you write or read something to or from a channel, the current position changes to the next character after what you just wrote or read. Occasionally, you may want to skip to a particular

File Manipulation

Gotchas

Don't forget to flush your out_channel s if you want to actually write something. This is particularly important if you are writing to non-files such as the standard output ( stdout ) or a socke

File Manipulation

Example

<!-- $MDX dir=examples --> We can compile and run this example: <!-- $MDX file=examples/file_manip.ml -->

Calling C Libraries

Calling C Libraries

MiniGtk

(Pop quiz: what happens if we need to define a class which is both a base class from which other classes can be derived, and is also a non-virtual class of which the user should be allowed to cre

Calling C Libraries

Wrapping Calls to C Libraries

In order for it to get passed to OCaml code at all, we must somehow convert it to a value . Luckily we can quite easily use the C API to create value blocks which the OCaml garbage collector

Calling Fortran Libraries

Calling Fortran Libraries

Step 1: Compile the Fortran Routine

Note that it's up to the caller to know that dens and temp are actually arrays. Failure to pass an array will cause a segmentation violation since the gtd6_ function is using them as arrays (

Calling Fortran Libraries

Step 2: Create the C Wrapper

The file must include the OCaml header files alloc.h , memory.h , and mlvalue.h . The function first calls the CAMLparam5 macro. This is required at the start of any function that uses the CAM

Calling Fortran Libraries

Step 3: Compile the Shared Library

This will create a shared object library called wrapper.so containing the fortran function and the wrapper function. The -lg2c option is required to provide the implementations of the built in fo

Calling Fortran Libraries

Step 4: Now to OCaml

And voila, we've called the fortran function from OCaml. prompt> ocamlc -o test gtd6.cmo wrapper.so prompt> ocamlc -c gtd6.ml At this point, the steps that are given are to compile this into byt

Using the OCaml Compiler Toolchain

Using the OCaml Compiler Toolchain

Compilation Basics

In this section, we will first see how to compile a simple program using only ocamlc or ocamlopt . Then we will see how to use libraries and how to take advantage of the findlib system, wh

Using the OCaml Compiler Toolchain

The ocamlc and ocamlopt Compilers

If your program depends upon third-party libraries, you must pass them on the command line. You must also indicate the libraries on which these libraries depend. You must also pass the -I option

Using the OCaml Compiler Toolchain

Using the ocamlfind Front-End

Separate compilation (one command for module1.ml , another for module2.ml and another to link the final output) is usually not performed manually but only when using an automated build system t

Using the OCaml Compiler Toolchain

Interlude: Making a Custom Toplevel

OCamlfind also supports ocamlmktop : We run toplevel and get an OCaml toplevel with modules Unix , Module1 , and Module2 all available, allowing us to experiment interactively with our p

Using the OCaml Compiler Toolchain

Dune: An Automated Build System

The dune quick-start guide shows you how to write such description files for more complicated situations, and how to structure, build, and run dune projects. The most popular modern system

Using the OCaml Compiler Toolchain

Bytecode Using Dune

We can also do that with .exe . The .bc stands for generic bytecode file, and it can be an executable or library. Finally, we compile and execute it. Next, create a main.ml file, ad

Using the OCaml Compiler Toolchain

Other Build Systems

OMake Another OCaml build system. GNU make GNU make can build anything, including OCaml. May be used in conjunction with OCamlmakefile Oasis Generates a configure, build, and install system from

Debugging

Debugging

Tracing Functions Calls in the Toplevel

The simplest way to debug programs in the toplevel is to follow the function calls, by “tracing” the faulty function:

Debugging

Polymorphic Functions

For our sorting routine, a single type constraint on the argument of the exchange function warranties a monomorphic typing, that allows a proper trace of function calls: A simple way to overc

Debugging

Limitations

Compiler builtins are described in the standard library documentation. At compile time, the __LOC__ builtin is substituted with its location in the program, described as a string "File %S, l

Debugging

The OCaml Debugger

ocamldebug runs on ocamlc bytecode programs (it does not work on native code executables), and it does not work under native Windows ports of OCaml (but it runs under the Cygwin port). We now

Debugging

Launching the Debugger

Then the debugger answers with a banner and a prompt: We launch the debugger: At runtime, the program raises an uncaught exception Not_found . Suppose we want to find where and why this e

Debugging

Finding the Cause of a Spurious Exception

To sum up: if you're developing a program you can compile it with the -g option, to be ready to debug the program if necessary. Hence, to find a spurious exception you just need to type ocamlde

Debugging

Getting Help and Info in the Debugger

To get more info about the current status of the debugger you can ask it directly at the toplevel prompt of the debugger; for instance:

Debugging

Setting Break Points

Now we can guess why List.assoc will fail to find "INRIA" in the list... Then, we can step and find what happens just before ( <|b|> ) List.assoc is about to be called in find_address : L

Debugging

Using the Debugger Under Emacs

Under Emacs you call the debugger using ESC-x ocamldebug a.out . Then Emacs will send you directly to the file and character reported by the debugger, and you can step back and forth using ES

Debugging

Printing a Back Trace for an Uncaught Exception

The environment variable OCAMLRUNPARAM also works when working on a program built with dune : From this back trace it should be clear that we receive a Not_found exception in List.assoc

Debugging

Detecting a Data Race with Thread Sanitizer

With the introduction of Multicore parallelism in OCaml 5, comes the risk of introducing data races among the involved Domain s. Luckily the Thread Sanitizer (TSan) mode for OCaml is helpful to

Debugging

Installing a TSan Switch

If the above fails during installation of conf-unwind with No package 'libunwind' found , try setting the environment variable PKG_CONFIG_PATH to point to the location of libunwind.pc , for

Debugging

Detecting a Data Race

The TSan instrumentation benefits from compiling programs with debug information, which happens by default under dune . To manually invoke the ocamlopt compiler under our 5.2.0+tsan switch

Error Handling

Error Handling

Errors as Special Values

The rest of this document presents and compares approaches towards error handling. Exceptions provide a means to deal with errors at the control flow level, while option and result make error

Error Handling

Exceptions

Here, we add a variant Foo to the type exn and create a function that will raise this exception. Now, how do we handle exceptions? The construct is try ... with ... : Exceptions belong to