Lwt user manual
Contents
1. The stub for creating the job external mkdir_job string gt int gt unix_mkdir job lut unix mEdir job The stub for reading the result of the job external mkdir_result unix_mkdir job gt unit lut unix mEdir result The stub reading the result of the job external mkdir_free unix_mkdir job gt unit lwt_unix_mkdir_free And finally the ocaml function let mkdir name perms Lwt_unix execute_job mkdir_job name perms mkdir_result mkdir_free 172. and patterng exprg in expr which is a parallel let binding construction For example in the following code lwt x f and y g in expr the thread f O and g are launched in parallel and their result are then bound to x and y in the expression expr Of course you can also launch the two threads sequentially by writing your code like that lwt x f in lwt y g O in expr e try_lwt expr with pattern gt expr finally expr which is the equivalent of the standard try with construction but for Lwt Both exception raised by Pervasives raise and Lwt fail are caught e for_lwt ident expr to downto CPT final do expr done init which is the equivalent of the standard for construction but for Lwt e raise lut ern which is the same as Lwt fail ezn but with backtrace support 2 3 1 Correspondence table You can keep in mind the following table to write code using lwt without Lwt let pattern expr and patierng expro and patternn exprn in expr try expr with pattern gt expry patterng gt expro patterny gt exprn for ident exprip to CPT final do expr done raise ern assert expr match expr with pattern gt expry patterno gt expro patternn gt exprn while expr do expr done 2 4 Backtrace support with Lwt lwt pattern espr and patierng expro and patternn exprn in expr try_lwt expr with pattern gt ex
3. Lwt state waiter HE HH HH HOH H HE 2 2 1 Primitives for thread composition The most important operation you need to know is bind val bind a Lwt t gt a gt b Lwt t gt b Lwt t bind t f creates a thread which waits for t to terminates then pass the result to f If t is a sleeping thread then bind t f will be a sleeping thread too until t terminates If t fails then the resulting thread will fail with the same exception For example consider the following expression Lwt bind Lwt_io read_line Lwt_io stdin fun str gt Lwt_io printlf You typed S str This code will first wait for the user to enter a line of text then print a message on the standard output Similarly to bind there is a function to handle the case when t fails val catch unit gt a Lwt t gt exn gt a Lwt t gt a Lwt t catch f g will call then waits for its termination and if it fails with an exception exn calls g exn to handle it Note that both exceptions raised with Pervasives raise and Lwt fail are caught by catch 2 2 2 Cancelable threads In some case we may want to cancel a thread For example because it has not terminated after a timeout This can be done with cancelable threads To create a cancelable thread you must use the Lwt task function val task unit gt a Lwt t a Lwt u It has the same semantic as Lwt wait except that the sleeping thread can be canceled with Lwt cancel val
4. and the prompt is a signal so it can change dynamically 7 Other libraries 7 1 Detaching computation to preemptive threads It may happen that you want to run a function which will take time to compute or that you want to use a blocking function that cannot be used in a non blocking way For these situations Lwt allow you to detach the computation to a preemptive thread This is done by the module Lwt_preemptive of the lwt preemptive package which maintains a spool of system threads The main function is val detach a gt b gt a gt b Lwt t detach f x will execute f x in another thread and asynchronously wait for the result The lwt extra package provides wrappers for a few blocking functions of the standard C library like gethostbyname in the module Lut 1ib 7 2 SSL support The package 1wt ssl provides the module Lut ssl which allow to use SSL asynchronously 7 3 Glib integration The lwt glib embed the glib main loop into the Lwt one This allow you to write GTK application using Lwt The one thing you have to do is to call Lwt_glib install at the beginning of you program 8 Writing stubs using Lwt 8 1 Thread safe notifications If you want to notify the main thread from another thread you can use the Lwt thread safe notification system First you need to create a notification identifier which is just an integer from the OCaml side using the Lwt_unix make notification function then you can send it from e
5. Lwt user manual J r mie Dimino November 4 2011 Contents 1 Introduction 2 The Lwt core library 2 1 LW concepts usso e A Re a e AES a pe ha da AeA Cee de fo 2 2 Primitives for thread creation LL 2 2 1 Primitives for thread composition LL 2 2 2 Cancelable threads coria a a a al a Le ges 2 2 3 Primitives for multi thread composition 0 000000008 22 4 Threads local storages el ua ete a a a oe oe Bo Ee bk ce a DDD A A ee ete ase er eg EL a 2 3 The syntax extension 96 304 ase tele ae Bh ea ke ee a da aa ee Hd 2 3 1 Correspondence table e 2A Backtrace SUPpott sears dadas daa ae ala de o we A eta es 2 5 Other modules of the core library LL La 25t MUERES acti P AS Geo P NGS GE AE OE O E O RR 23918 Data streams hace ae ek oe Ge AA i ec a om ae a 2 9 4 Mailbox vatlables a eek dE Rd a a a eR Running a Lwt program The lwt unix library 41 Unig primitives era A ee ee Ed PE ee ee PR EE es 432 The Lwt scheduler d ous GEN hae DE P dr ea Pe et he doe eh dd A 4 3 The loggina facility Ser aoc a Dio De EE DE BSE RGE e eee dl ee eg Be a The Lwt react library The lwt text library 0 1 Texechannels od G54 as dd dd dl a ela ala a a Fr btt dd 6 2 Terminal wtilities s ateate at a dE Metges ul gla a ae ad ES 6 84 Readzline ep SE LG P Pa PA ed ELENA Le RAL US Other libraries 7 1 Detaching computation to preemptive thread
6. cancel a Lwt t gt unit The thread will then fails with the exception Lwt Canceled To execute a function when the thread is canceled you must use Lwt on_cancel val on_cancel a Lwt t gt unit gt unit gt unit Note that it is also possible to cancel a thread which has not been created with Lwt task In this case the deepest cancelable thread connected with the given thread will be cancelled For example consider the following code let waiter wakener Lwt task val waiter _a Lwt t lt abstr gt val vaRener a Lwt u lt abstr gt let t bind waiter fun x gt return x 1 val t int Lwt t lt abstr gt Here cancelling t will in fact cancel waiter t will then fails with the exception Lut Canceled Lwt cancel t unit Lwt state waiter int Lwt state Fail Lwt Canceled Lwt state t int Lwt state Fail Lwt Canceled By the way it is possible to prevent a thread from being canceled by using the function Lwt protected val protected a Lwt t gt a Lwt t Canceling proctected t will have no effect on t 2 2 3 Primitives for multi thread composition We now show how to compose several threads at the same time The main functions for this are in the Lwt module join choose and pick The first one join takes a list of threads and wait for all of them to terminate val join unit Lwt t list gt unit Lwt t Moreover if at least one
7. char oc a unit Lwt t lt abstr gt Lwt state t char Lwt state Return a So after we write something the reading thread has been wakeup and has returned the value a 2 2 Primitives for thread creation There are several primitives for creating Lwt threads These functions are located in the module Lwt Here are the main primitives e Lut return a gt a Lwt t creates a thread which has already terminated and returned a value e Lwt fail exn gt a Lwt t creates a thread which has already terminated and failed with an exception e Lwt wait unit gt a Lwt t a Lwt u creates a sleeping thread and returns this thread plus a wakener of type a Lwt u which must be used to wakeup the sleeping thread To wake up a sleeping thread you must use one of the following functions e Lwt wakeup a Lwt u gt a gt unit wakes up the thread with a value e Lwt wakeup_exn a Lwt u gt exn gt unit wakes up the thread with an exception Note that this is an error to wakeup two times the same threads Lwt will raise Invalid_argument if you try to do so With these informations try to guess the result of each of the following expression Lwt state Lwt return 42 Lwt state fail Exit let waiter wakener Lwt wait Lwt state waiter Lwt wakeup wakener 42 Lwt state waiter let waiter wakener Lwt wait Lwt state waiter Lwt wakeup_exn wakener Exit
8. cking while it is being executed the whole program will be blocked and other events will not be handled until it returns Now let s look at the lwt equivalent Lwt_io read_char Lwt_io input_channel gt char Lwt t lt fun gt As you can see it does not returns a character but something of type char Lwt t The type a Lwt t is the type of threads returning a value of type a Actually the Lwt_io read_char will try to read a character from the given input channel and immediatly returns a light weight thread Now let s see what we can do with a Lwt thread The following code create a pipe and launch a thread reading on the input side let ic oc Lwt_io pipe val ic Lwt_io input_channel lt abstr gt val oc Lwt_io output_channel lt abstr gt let t Lwt_io read_char ic val t char Lwt t lt abstr gt We can now look at the state of our newly created thread Lwt state t char Lwt state Sleep A thread may be in one of the following states e Return x which means that the thread has terminated successfully and returned the value x e Fail exn which means that the thread has terminated but instead of returning a value it failed with the exception exn e Sleep which means that the thread is currently sleeping and have not yet returned a value or an exception The thread t is sleeping because there is currently nothing to read on the pipe Let s write something Lwt_io write_
9. e is taken Taking an element from an empty mvar will block until one is added Mailbox variables are commonly used to pass messages between threads Note that a mailbox variable can be seen as a pushable stream with a limited memory 3 Running a Lwt program Threads you create with Lwt always have the type Lwt t If you want to write a program and run it this is not enough Indeed you don t know when a Lwt thread is terminated For example if your program is just let _ Lwt_io printl Hello world you have no guarantee that the thread writing Hello world on the terminal will be terminated when the program exit In order to wait for a thread to terminate you have to call the function Lwt_main run val Lwt_main run a Lwt t gt a This functions wait for the given thread to terminate and returns its result In fact it does more than that it also run the scheduler which is responsible for making thread to progress when events are received from the outside world So basically when you write a Lwt program you must call at the toplevel the function Lwt_main run For instance let Lwt_main run Lwt_io printl Hello world Note that you must call Lwt_main run only once at a time It cannot be used anywhere to get the result of a thread It must only be used in the entry point of your program 4 The lwt unix library The package 1wt unix contains all unix dependant modules of Lwt Among all its features it implem
10. ents cooperative versions of functions of the standard library and the unix library 11 4 1 Unix primitives The Lwt_unix provides cooperative system calls For example the Lwt counterpart of Unix read is val read file_descr gt string gt int gt int gt int Lwt t Lut io provides features similar to buffered channels of the standard library of type in channel or out channel but cooperatively Lwt_gc allow you to register finaliser that return a thread At the end of the program Lwt will wait for all the finaliser to terminates 4 2 The Lwt scheduler Threads doing IO may be put asleep until some events are received by the process For example when you read from a file descriptor you may have to wait for the file descriptor to become readable if no data are immediatly available on it Lwt contains a shceduler which is responsible for managing multiple threads waiting for events and restart them when needed This scheduler is implemented by the two modules Lwt_engine and Lwt_main Lwt_engine is a low level module it provides signatures for IO multiplexers as well as several builtin imple mentation Lwt support by default multiplexing IO with libev or Unix select The signature is given by the class Lwt_engine t libev is used by default on Unix because it supports any number of file descriptors while Unix select supports only 1024 at most and is also much more efficient On Windows Unix select is used because libev does no
11. f section somethign happend s msg else Lwt return The advantages of using the syntax extension are the following e it check the log level before calling the logging function so arguments are not computed if not needed e debugging logs can be removed at parsing time By default the syntax extension remove all logs with the level debug To keep them pass the command line option lwt debug to camlp4 5 The Lwt react library The Lwt_react module provide helpers for using the react library with Lwt It extends the React module by adding Lwt specific functions It can be used as a replacement of React For example you can add at the beginning of you program open Lwt_react instead of open React or module React Lwt_react Among the added functionnality we have Lwt_react E next which takes an event and returns a thread which will wait until the next occurence of this event For example open Lwt_react let event push E create val event _a React event lt abstr gt val push _a gt unit lt fun gt let t E next event val t _a Lwt t lt abstr gt Lwt state t i _a Lwt state Sleep push 42 unit Lwt state t int Lwt state Return 42 Another interesting feature is the ability to limit events resp signals to occurs resp to changes too often For example suppose you are doing a program which displays something on the screeen each time a signa
12. functions over lists similar to the ones of the List module but using functions that return a thread For example val iter_s a gt unit Lwt t gt a list gt unit Lwt t val iter_p Ca gt unit Lwt t gt a list gt unit Lwt t In iter_s f 1 iter_s will call f on each elements of 1 waiting for completion between each elements On the contrary in iter p f 1 iter p will call f on all elements of 1 then wait for all the threads to terminate 2 5 3 Data streams Lwt streams are used in a lot of places in Lwt and its sub libraries They offer a high level interface to manipulate data flows A stream is an object which returns elements sequentially and lazily Lazily means that the source of the stream is guessed for new elements only when needed This module contains a lot of stream transformation iteration and scanning functions The common way of creating a stream is by using Lwt_stream from or by using Lwt_stream create val from unit gt a option Lwt t gt a Lwt_stream t val create unit gt a Lwt_stream t a option gt unit As for streams of the standard library from takes as argument a function which is used to create new elements create returns a function used to push new elements into the stream and the stream which will receive them For example let stream push Lwt_stream create val stream _a Lwt_stream t lt abstr gt val push _a option gt unit lt f
13. ither the OCaml code with Lut unix send notification function or from the C code using the function lwt_unix_send_notification defined in lwt_unix_ h Notification are received and processed asynchronously by the main thread 15 8 2 Jobs For operations that can not be executed asynchronously Lwt uses a system of jobs that can be executed in a different threads A job is composed of four functions e A function to create the job which creates a job structure info and stores parameters in it This function is executed in the main thread e A function which execute the job This one may be executed asynchronously in another thread e A function which read the result of the job This function is executed in the main thread e And finally a function that free resources allocated for the job which is also executed in the main thread We show as example the implementation of Lwt_unix mkdir On the C side we have struct job_mkdir 4 Informations required by lvt It must be the first field of the structure struct lwt_unix_job job The name of the directory to create char name Permissions for the directory int perms The result of the call to mkdir int result The errno value after the call int error_code 3 Convenient macro for retrieving a mkdir job info structure from an ocaml custom value define Job_mkdir_val v struct job_mkdir Data_custom_val v The function
14. l changes If at some point the signal changes 1000 times per second you probably want not to render it 1000 times per second For that you use Lwt_react S limit val limit unit gt unit Lwt t gt a React signal gt a React signal Lwt_react S limit f signal returns a signal which varies as signal except that two consecutive updates are separeted by a call to f For example if f returns a thread which sleep for 0 1 seconds then there will be no more than 10 changes per second For example 13 open Lwt_react let draw x Draw the screen let The signal we are interested in let signal in The limited signal let signal S limit fun gt Lwt_unix sleep 0 1 signal in Redraw the screen each time the limited signal change S notify_p draw signal 6 The lwt text library The lwt text library provides functions to deal with text mode in a terminal It is composed of the three following modules e Lwt_text which is the equivalent of Lwt_io but for unicode text channels e Lwt_term providing various terminal utilities such as reading a key from the terminal e Lwt_read line which provides functions to input text from the user with line editing support 6 1 Text channels A text channel is basically a byte channel plus an encoding Input resp output text channels decode resp encode unicode characters on the fly By default output text channels use transliteration
15. ng it right with threads is a hard job Moreover system threads consume non negligible resources and so you can only launch a limited number of threads at the same time Thus this is not a real solution Lwt offers a new alternative It provides very light weight cooperative threads launching a thread is a very quick operation it does not require a new stack a new process or anything else Moreover context switches are very fast In fact it is so easy that we will launch a thread for every system call And composing cooperative threads will allow us to write highly asynchronous programs In a first part we will explain the concepts of Lwt then we will describe the many sub libraries of Lwt 2 The Lwt core library In this section we describe the basics of Lwt It is advised to start an ocaml toplevel and try the given code examples To start launch ocaml in a terminal or in emacs with the tuareg mode and type use topfind require lut Lwt is also shipped with an improved toplevel which supports line edition and completion If it has been correctly installed you should be able to start it with the following command lwt toplevel 2 1 Lwt concepts Let s take a classical function of the Pervasives module Pervasives input_char in_channel gt char lt fun gt This function will wait for a character to come on the given input channel then return it The problem with this function is that it is blo
16. pry patterng gt expro patterny gt exprn for_lwt ident exprjn 4 to ESPT final do expr done raise_lwt egn assert_lwt expr match_lwt expr with pattern gt expry patterng gt expro patternn gt exprn while_lwt expr do expr done When using Lwt exceptions are not recorded by the ocaml runtime and so you can not get backtraces However it is possible to get them when using the syntax extension All you have to do is to pass the lwt debug switch to camlp4 ocamlfind ocamlc syntax camlp4o package lwt syntax ppopt lwt debug linkpkg o foo foo ml 2 5 Other modules of the core library The core library contains several modules that depend only on Lwt The following naming convention is used in Lwt when a function takes as argument a function returning a thread that is going to be executed sequentially it is suffixed with _s And when it is going to be executed in parallel it is suffixed with _p For example in the Lwt_list module we have val map_s a gt b Lwt t gt a list gt b list Lwt t val map_p a gt b Lwt t gt a list gt b list Lwt t 2 5 1 Mutexes Lwt_mutex provides mutexes for Lwt Its use is almost the same as the Mutex module of the thread library shipped with OCaml In general programs using Lwt do not need a lot of mutexes They are only usefull for serialising operations 2 5 2 Lists The Lwt_list module defines iteration and scanning
17. s Qu LL LL Le T2 COOL SUppOLt eG cde a 4a da do Pre Gok SOE Gee ee Bee ee ee Bek See de lo op Glib integration eva a ts OE hoe P Ed eR Pee EA del wm OD 000000 ooaoa AW 11 11 12 12 12 13 14 14 14 15 8 Writing stubs using Lwt 8 1 Thread safe notifications 8 2 JODS e dia rs a es 1 Introduction When writing a program a common developer s task is to handle IO operations Indeed most software interact with several different resources such as e the kernel by doing system calls e the user by reading the keyboard the mouse or any input device e a graphical server to build graphical user interface e other computers by using the network When this list contains only one item it is pretty easy to handle However as this list grows it becomes harder and harder to make everything works together Several choices have been proposed to solve this problem e using a main loop and integrate all components we are interacting with into this main loop e using preemptive system threads Both solution have their advantages and their drawbacks For the first one it may works but it becomes very complicated to write some piece of asynchronous sequential code The typical example being with graphical user interfaces freezing and not redrawing themselves because they are waiting for some blocking part of the code to complete If you already wrote code using preemptive threads you shall know that doi
18. so they will not fails because text you want to print cannot be encoded in the system encoding For example with you locale sets to C and the variable name set to J r mie you got lwt Lut text printl My name is s name My name is J r mie 6 2 Terminal utilities The Lwt_term allow you to put the terminal in raw mode meanings that input is not buffered and character are returned as the user type them For example you can read a key with lwt key Lwt_term read_key val key Lwt_term key Lwt_term Key_control j The second main feature of Lwt_term is the ability to prints text with styles For example to print text in bold and blue open Lwt_term lwt printlc fg blue bold text foo foo If the output is not a terminal then printlc will drop styles and act as Lwt_text printl 14 6 3 Read line Lwt_read_line provides a full featured and fully customisable read line implementation You can either use the high level and easy to use read_ functions or use the advanced Lwt_read_line Control read_ functions For example open Lwt_term lwt 1 Lut read line read line prompt text foo gt foo gt Hello world val 1 Text t Hello world The second class of functions is a bit more complicated to use but allow to control a running read line instances For example you can temporary hide it to draw something you can send it commands fake input
19. t works properly The user may change at any time the backend in use The engine can also be used directly in order to integrate other libraries with Lwt For example GTK need to be notified when some events are received If you use Lwt with GTK you need to use the Lwt scheduler to monitor GTK sources This is what is done by the lwt glib package The Lwt_main module contains the main loop of Lwt It is run by calling the function Lwt_main run val Lwt_main run a Lwt t gt a This function continously run the scheduler until the thread passed as argument terminates 4 3 The logging facility The package lwt unix contains a module Lwt_log providing loggers It support logging to a file a channel or to the syslog daemon You can also defines your own logger by providing the appropriate functions function Lwt_log make Several loggers can be merged into one Sending logs on the merged logger will send these logs to all its components For example to redirect all logs to stderr and to the syslog daemon Lwt_log default_logger Lwt_log broadcast Lwt_log channel close_mode Keep channel Lwt_io stderr Lwt_log syslog facility User Lwt also provides a syntax extension in the package lwt syntax log It does not modify the language but it replaces log statement of the form Lwt_log info_f section something happened s msg by 12 if Lwt_log Section level section lt Lwt_log Info then Lwt_log info_
20. that effectively executes the job static void worker_mkdir struct job_mkdir job Call mkdir and save its result job gt result mkdir job gt name job gt perms Save the contents of errno job gt error_code errno The stub that create the job CAMLprim value lwt_unix_mkdir_job value val_name value val_perms struct job_mkdir job lwt_unix_new struct job_mkdir 16 Sets the worker for this job job gt job worker lwt_unix_job_worker worker_mkdir Copy the name of the directory into the C memory job gt name lwt_unix_strdup String_val val_name Copy the perms parameter job gt perms Int_val val_perms Put the job into an ocaml custom value and returns it return lwt_unix_alloc_job amp job gt job The stub that read the result of the job CAMLprim value lwt_unix_mkdir_result value val_job struct job_mkdir job Job_mkdir_val val_job If mkdir failed raise the unix error now if job gt result lt 0 unix_error job gt error_code mkdir Nothing return Val_unit The stub that free resources CAMLprim value lwt_unix_mkdir_free value val_job struct job_mkdir job Job_mkdir_val val_job Free the name of the directory free job gt name Free resources allocated by lwt_unix for this job lwt_unix_free_job amp job gt job return Val_unit and on the ocaml side
21. thread fails join 1 will fails with the same exception as the first to fail after all threads threads terminated On the contrary choose waits for at least one thread to terminate then returns the same value or exception val choose a Lwt t list gt a Lwt t For example let waiterl wakeneri Lwt wait val vaiterl _a Lwt t lt abstr gt val vaReneri _a Lwt u lt abstr gt let waiter2 wakener2 Lwt wait val waiter2 _a Lwt t lt abstr gt val vaRener _a Lwt u lt abstr gt let t Lwt choose waiter1l waiter2 val t _a Lwt t lt abstr gt Lwt state t i _a Lwt state Sleep Lwt wakeup wakener2 42 unit Lwt state t int Lwt state Return 42 Thel last one pick is the same as join except that it cancels all other threads when one terminates 2 2 4 Threads local storage Lwt can stores variables with different values on different threads This is called threads local storage For example this can be used to store contexts or thread identifiers The contents of a variable can be read with val Lwt get a Lwt key gt a option which takes a key to identify the variable we want to read and returns either None if the variable is not set or Some x if it is The value returned is the value of the variable in the current thread New keys can be created with val Lwt new_key unit gt a Lwt key To set a variable you must use
22. un gt push Some 1 unit push Some 2 unit push Some 3 unit Lwt state Lwt_stream next stream int Lwt state Return 1 Lwt state Lwt_stream next stream int Lwt state Return 2 Lwt state Lwt_stream next stream int Lwt state Return 3 Lwt state Lwt_stream next stream int Lwt state Sleep Note that streams are consumable Once you take an element from a stream it is removed from it So if you want to iterates two times over a stream you may consider clonning it with Lwt_stream clone Cloned 10 stream will returns the same elements in the same order Consuming one will not consume the other For example let s Lwt_stream of_list 1 2 val s int Lwt_stream t lt abstr gt let s Lwt_stream clone s val s int Lwt_stream t lt abstr gt Lwt state Lwt_stream next s int Lwt state Return 1 Lwt state Lwt_stream next s int Lwt state Return 2 Lwt state Lwt_stream next s int Lwt state Return 1 Lwt state Lwt_stream next s int Lwt state Return 2 2 5 4 Mailbox variables The Lwt_mvar module provides mailbox variables A mailbox variable also called a mvar is a cell which may contains 0 or 1 element If it contains no elements we say that the mvar is empty if it contains one we say that it is full Adding an element to a full mvar will block until on
23. val Lwt with_value a Lwt key gt a option gt unit gt b gt b with_value key value f will executes f with the binding key gt value The old value associated to key is restored after f terminates For example you can use local storage to store thread identifiers and use them in logs let id_key Lwt new_key let log msg let thread_id match Lwt get id_key with Some id gt id None gt main in Lwt_io printlf s s thread id msg lwt Lwt join Lwt with_value id_key Some thread 1 fun gt log foo Lwt with_value id_key Some thread 2 fun gt log bar 2 2 5 Rules Lwt will always try to execute the more it can before yielding and switching to another cooperative thread In order to make it works well you must follow the following rules e do not write function that may takes time to complete without using Lwt e do not do IOs that may block otherwise the whole program will hang You must instead use asyn chronous IOs operations 2 3 The syntax extension Lwt offers a syntax extension which increases code readability and makes coding using Lwt easier To use it add the Iwt syntax package when compiling ocamlfind ocamlc syntax camlp4o package lwt syntax linkpkg o foo foo ml Or in the toplevel after loading topfind camlp4o require lut syntax The following construction are added to the language e lwt pattern expr
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