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1. Labelled and Optional Arguments

   Passing Labelled Arguments Using the Pipe Operator

   Let's modify the range function previously defined by adding an additional parameter step . Declaring a function's unlabelled argument as the first one simplifies reading the

   Introduction

2. Mutability and Imperative Control Flow

   Remark: References Are Single Field Records

   create does the same as the ref function provided by the standard library. assign does th [...] deref does the same as the ( ! ) operator. The functions: Since references are single field records, we can define

   Introduction

3. Mutability and Imperative Control Flow

   Semicolon

   er containing value n Updates the reference's contents to 2 × (n + 1) Imagine we want to write a function that: In OCaml, begin … end and parentheses [...] olon is not truly an operator because it is not a function of type unit -> 'a -> 'a . It is rather a construct of the language. It allows adding a semicolo

   Introduction

4. Arrays

   Introduction

   ch as: This tutorial aims to introduce the subject of arrays in OCaml and showcase the most useful functions and use cases. Despite these differences, many of the functions readily available on arrays are similar to the ones available for lists. Please refer to the List

   Data Structures

5. Arrays

   Creating Arrays

   Array.init generates an array of a given length by applying a function to each index of the array, starting at 0. The fo [...] array containing the first 5 even numbers using a function which doubles its argument: Alternatively, you can create an array using the Array.make

   Data Structures

6. Arrays

   Copying Part of an Array into Another Array

   tarting at index 1 into the last part of zeroes , starting at index 3 . We can also use this function to copy part of an array onto itself: This copie [...] hin the bounds of each array. The Array.blit function efficiently copies a contiguous part of an array into an array. Similar to the array.(x) <- y ope

   Data Structures

7. Maps

   Introduction

   StringMap , OCaml's toplevel displays the module's signature. Since it contains a large number of functions, the output copied here is shortened for brevi [...] es the keys' type to be used in the maps, and a function for comparing them. Note : The concept of a Map in this tutorial refers to a data structure that

   Data Structures

8. Maps

   Finding Entries in a Map

   ote that find_first_opt and find_last_opt return the key-value pair, not just the value. The functions find_first and find_last behave similarly, e [...] d find_last_opt if we want to use a predicate function: find_opt returns None find throws the Not_found exception When the searched key is absent fr

   Data Structures

9. Maps

   Changing the Value Associated With a Key

   You should experiment with different update functions; several behaviors are possible. To change a key's associated value, use the update function. It takes a key, a map, and an update

   Data Structures

10. Sets

    Sets With Custom Comparators

    e Insensitive String Set"). We can accomplish this by passing an ad-hoc module to the Set.Make function: Let's say we want to create a set of strings th [...] et module we created uses the built-in compare function provided by the String module. The Set.Make functor expects a module with two definitions: a t

    Data Structures

11. Sequences

    Constructing Sequences

    tains a module for sequences called Seq . It contains Seq.int , which we implemented above. The function ints n looks as if building the infinite sequen [...] in_int . We can also construct sequences using functions. Here is how to build an infinite sequence of integers: Note: The second component of each Se

    Data Structures

12. Sequences

    Sequences for Conversions

    Similar functions are also provided for sets, maps, hash tables ( [...] , it is advised to expose to_seq and of_seq functions. Lists Arrays Strings Throughout the OCaml Standard Library, sequences are used as a bri

    Data Structures

13. Sequences

    Miscellaneous Considerations

    ps it would be enlightening to illustrate the use of Seq.unfold by re-implementing the already seen function primes? Perhaps in an exercise rather than in the [...] accumulator, and also the fact that the producer function loops until it finds a new prime to yield, because Seq.unfold does not allow the producer to “ski

    Data Structures

14. Monads

    Monad Laws

    e the right type to be on the right-hand side of >>= . So we have to insert an extra anonymous function fun x -> ... to make the types correct. (m >>= [...] ing the trivial effect on a value, then binding a function on it, is the same as just calling the

    Data Structures

15. Memory Representation of Values

    Polymorphic Variants

    ts with constructors thus use one word of memory more than normal variant constructors. The hash function is designed to give the same results on 32-bit an [...] integer value is determined by applying a hash function to the name of the variant. The hash

    Runtime & Compiler

16. Understanding the Garbage Collector

    Attaching Finalizer Functions to Values

    <div class="note"> OCaml's automatic memory management guarantees that a value will eventually be freed when it's no longer in use, either via the GC sweeping it or the program terminating. It'

    Runtime & Compiler

17. The Compiler Backend: Bytecode and Native code

    Executing Bytecode

    ces bytecode that targets the standard OCaml runtime by default, and so needs to know about any C functions that are referenced from other libraries that [...] s: the bytecode interpreter, GC, and a set of C functions that implement the primitive operations. The bytecode contains instructions to call these C

    Runtime & Compiler

18. 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 fortran

    Tutorials

19. Error Handling

    Predefined Exceptions

    reexisting exceptions Raise custom exceptions When implementing a software component which exposes functions raising exceptions, a design decision must be [...] ones are intended to be raised by user-written functions: Although the last one doesn't look as an exception, it actually is. The standard library pr

    Guides

20. Error Handling

    Printing

    own exceptions: To print an exception, the module Printexc comes in handy. For instance, the function notify_user : (unit -> 'a) -> 'a below calls a function, and if it fails, prints the exception on stderr . If stack traces are enabled, this

    Guides

21. Error Handling

    Throwing Exceptions From option or result

    ons, pattern matching and raise must be used. From option to Invalid_argument exception, use function Option.get : From result to Invalid_argument exception, use functions Result.get_ok and Result.get_error : This is done by using the following

    Guides

22. Error Handling

    Conversion Between option and result

    From option to result , use function Option.to_result : From result to option , use function Result.to_option : This is done by using the following

    Guides

23. OCaml Programming Guidelines

    Factor out snippets of repeated code by defining them in separate functions

    Sharing code obtained in this way facilitates maintenance, since every correction or improvement automatically spreads throughout the program. Besides, the simple act of isolating and naming a sn

    Resources

24. OCaml Programming Guidelines

    Never copy-paste code when programming

    ten lines of code is repeated twenty times throughout the program. By contrast, if an auxiliary function defines those ten lines, it is fairly easy to see [...] find where those lines are used: simply where the function is called. If code is copy-pasted all over the place, then the program is more difficult to und

    Resources

25. OCaml Programming Guidelines

    Always give the same name to function arguments which have the same meaning

    If necessary, make this nomenclature explicit in a comment at the top of the file. If there are several arguments with the same meaning, then attach numeral suffixes to them.

    Resources

26. OCaml Programming Guidelines

    Local identifiers can be brief and should be reused from one function to another

    A tolerated exception to the recommendation not to use capitalisation to separate words within identifiers when interfacing with existing libraries which use this naming convention. This lets OCa

    Resources

27. OCaml Programming Guidelines

    Exceptions

    -> Handling all exceptions by try ... with _ -> is usually reserved for the program's main function. If you must catch every exception in order to [...] e system as possible. For example, every search function that fails should raise the predefined exception Not_found . Be careful to handle the exceptions

    Resources

28. OCaml Programming Guidelines

    How to Choose Between Classes and Modules

    Use modules when the data structures are fixed and their functionality is equally fixed or it's enough to add new functions in the programs which use them. Use conventional data structures (in particular, variant type

    Resources

29. OCaml Programming Guidelines

    Clarity of OCaml Code

    r programming paradigms): imperative programming (based on the notion of state and assignment), functional programming (based on the notion of function,

    Resources

30. OCaml Programming Guidelines

    Function application: same rules as for trigonometric functions

    In mathematics you write sin x to mean sin (x) . In the same way sin x + cos x means (sin x) + (cos x) not sin (x + (cos x)) . Use the same conventions in OCaml: write f x + g x to mean

    Resources

31. Basic Data Types and Pattern Matching

    Byte Sequences

    rray . Operations on bytes values are provided by the Stdlib and the Bytes modules. Only the function Bytes.get allows direct access to the character [...] bytes literally, so they must be produced by a function. <!--CR Moved intro pp to after example, as it contains the definition. Perhaps a short intro se

    Introduction

32. Basic Data Types and Pattern Matching

    Lists

    ce with respect to lists: Operations on lists are provided by the List module. The List.append function concatenates two lists. It can be used as an oper [...] values they contain. Lists play a central role in functional programming, so they have a dedicated tutorial . As literals, lists are very much like arrays

    Introduction

33. Basic Data Types and Pattern Matching

    Tuples

    t associative . In the standard library, both are defined using pattern matching. Here is how a function extracts the third element of the product of four types: The predefined function fst returns the first element of a pair, while snd returns the second element of a pair. This

    Introduction

34. Basic Data Types and Pattern Matching

    Recursive Variants

    bol _ , which catches everything. It returns false on all data that is neither Array nor Object . Functions defined using pattern matching on recursive variants are often recursive too. This function checks if a name is present in a whole JSON tree: Both constructors Array and Object contain values of type json . This is the case for the following definition, which can be used to store J

    Introduction

35. Basic Data Types and Pattern Matching

    Revisiting Predefined Types

    In the end, the only type construction that does not reduce to a variant is the function arrow type. Pattern matching allows the inspection of values of any type, except functions. Even integers and floats can be seen as enumerated-like variant types, with many constructors an

    Introduction

36. Basic Data Types and Pattern Matching

    User-Defined Polymorphic Types

    ors. Here are the terms applicable to data types: In the OCaml community, as well as in the larger functional programming community, the word polymorphism [...] ssary to distinguish them. Here is how the map function can be defined in this type: It can be used to represent arbitrarily labelled binary trees. Assu

    Introduction

37. Loops and Recursions

    For Loops and While Loops

    with for loops, the language doesn't provide a way to break out of a while loop, except by throwing an exception, so this means that while loops have fairly limited use. Again, remember that functional programmers like recursion, so while loops are second-class citizens in OCaml. <!-- $MDX skip --> While loops in OCaml are written: Functional programmers tend to use recursion instead

    Introduction

38. Loops and Recursions

    Looping Over Strings

    String.copy copies a string, like strdup . There is also a String.iter function which works like List.iter , except over the cha [...] e also contains dozens of useful string-related functions, and some of them are concerned with looping over strings.

    Introduction

39. Lists

    Association Lists

    e able to make a list of pairs from a pair of lists and vice versa. The List module provides the functions split and combine for this purpose: Ass [...] library's Map or Hashtbl modules. But these functions from the List module are useful for lists which are generally small, and have other advantages

    Introduction

40. Labelled and Optional Arguments

    Forwarding an Optional Argument

    ?len is sufficient to forward without unwrapping. In the definitions of take and rtake , the function sub is called with optional arguments passed wi [...] ithout unwrapping. These examples reuse the sub function defined in the Defining Optional Parameters Without Default Values section.

    Introduction

41. Mutability and Imperative Control Flow

    Byte Sequences

    char array . The former should always be preferred, except when array is required by polymorphic functions handling arrays. updatable strings that can't b [...] ces can be created from string values using the function Bytes.of_string . Individual elements in the sequence can be updated or read by their index using

    Introduction

42. Mutability and Imperative Control Flow

    Breaking Loops Using Exceptions

    d. When the ASCII Escape character is read, the Exit exception is thrown, which terminates the iteration and displays the REPL reply: - : unit = () . The following example uses the get_char function we defined earlier (in the section Example: get_char Function ). Throwing the Exit exception is a recommended way to immediately exit from a loop.

    Introduction

43. Mutability and Imperative Control Flow

    References Inside Closures

    nt value. Similarly, calling c2 () will update its own independent counter. First, we define a function named create_counter that takes no arguments. I [...] sing !counter . In the following example, the function create_counter returns a closure that “hides” a mutable reference counter . This closure cap

    Introduction

44. Mutability and Imperative Control Flow

    Bad: Side Effects Depending on Order of Evaluation

    itialising record fields. Here, it is illustrated on a tuple value: Since the evaluation order for function arguments in OCaml is not explicitly defined, the [...] rintf "%s %s %s" is applied to the results. The function id_print returns its input unchanged. However, it has a side effect: it first prints the string i

    Introduction

45. Modules

    Module Inclusion

    es a module Extlib.List that has everything the standard List module has, plus a new uncons function. In order to override the default List module [...] ib at the beginning. Let's say we feel that a function is missing from the List module, but we really want it as if it were part of it. In an extlib.

    Module System

46. Functors

    Introduction

    his tutorial are available as a Git repo . As suggested by the name, a functor is almost like a function. However, while the inputs and outputs of functions are values, functors are between modules. A functor has a module as a parameter and returns a modu

    Module System

47. Functors

    Dependency Injection

    .Make(Int) . The module IterPrint is refactored into a functor that takes a module providing the function iter as a parameter. The with type 'a t := 'a [...] cy. At its compilation time, no implementation of function iter is available yet. iterPrint.ml Here is a refactoring of the module IterPrint to use dep

    Module System

48. First-Class Modules

    Introduction

    : Modules , Functors . First-class modules let you treat modules as values. You can pass them to functions, return them from functions, and store them in data structures. This provides an alternative to functors for some use cases an

    Module System

49. Options

    Exceptions vs Options

    dling for a longer discussion on error handling using options, exceptions, and other means. The function Sys.getenv : string -> string from the OCaml st [...] e variable is not defined. On the other hand, the function Sys.getenv_opt : string -> string option does the same, except it returns None if the variable

    Data Structures

50. Options

    Peel Off Doubly Wrapped Options

    itional examples demonstrating how Option.join works in different scenarios: Our custom join function is the same as the standard library's Option.joi [...] in multiple options. Let's define a custom join function of type 'a option option -> 'a option that peels off one layer from a doubly wrapped option:

    Data Structures