In this post I continue going through Chapter 1 of Real World OCaml. I learnt that:

- Errors may be caught at
**compile time**or at**run time**. Compile-time errors are preferred to runtime errors, because it’s better to catch errors as early as possible in the development process. An e.g. of a compile-time error is**type error**.**Exceptions**like division by zero is a runtime error. - Concatenation operators are provided as part of the
module, which is automatically opened in every OCaml program.**Pervasives** - We used the
operator for concatenating strings.**^** - One can define an
**anonymous function**using thekeyword. Anonymous functions don’t need to be explicitly named.*fun* - Another syntax for pattern matching in OCaml is the
construct. You can extract values from a data structure. E.g., you can extract the components of a tuple:*let*pattern*=*expression

123let a_tuple = (3,"three");;let (x,y) = a_tuple;;

OCaml put the value**3**in the variable, and**x****“three”**in the variable.*y*

### More on Lists

* Base* comes with a

*module that has a rich collection of functions for working with lists. We can access values from within a module by using*

**List****dot**notation. E.g.

compute the length of a list.**List.length**takes two arguments:**List.map****a list**and**a function for transforming the elements of that list**. It returns a new list with the transformed elements and does not modify the original list. The arguments of List.map do not have to be ordered because the function is passed on with**a labeled argument**. That is, the following are equivalent:*~f*

1List.map ~f:FunctionName Listname

1List.map ListName ~f:FunctionNametakes a list and a function as arguments, and checks if there are any elements of the list for which the provided function evaluates to**List.exists**.**true**

The common OCaml idiom uses * hd* to refer to the head of the list and

*to refer to the tail.*

**tl**### Options

An option is a data structure that is used to express that a value might or might not be present. It can be thought of as a list that have either zero or one element. Option is the standard way in OCaml to encode a value that might not be there. ** Some** and

**are operators (constructors) for options. E.g.**

*None*
1 2 |
let divide x y = if y = 0 then None else Some (x / y) ;; |

The function * divide* takes 2 arguments of type

*, and returns type*

**int***. The type signature is:*

**int option**##### val : divide int -> int -> int -> option = <fun>

The function * divide* either returns

*if the divisor is zero, or*

**None***of the result of the division otherwise.*

**Some**### Making Your Own Types: Records

You can define your own data type in OCaml using the following construct:

**type NewTypeName = { data structure with type information } ;;**

E.g. to define the type * point2d* with the two components of type

*:*

**float**
1 |
type point2d = { x : float; y : float };; |

point2d is of type * record*. You can think of record as a tuple where the individual fields are named, rather than being defined positionally. In this case, the first field is named

*, and the second is named*

**x***. You can construct a variable of type record with*

**y***, as usual:*

**let**
1 |
let p = { x = 3.; y = -4. };; |

The type-signature of p is

##### val p : point2d = {x = 3.; y = -4.}

You can access the content of a record type in many ways:

- Using pattern matching:

12let magnitude { x = x_pos; y = y_pos } =Float.sqrt (x_pos **. 2. +. y_pos **. 2.);;

The pattern match here binds the variableto the value contained in the*x_pos*field, and the variable*x*to the value in the*y_pos*field.*y* - Using
**field pruning**:

1let magnitude { x; y } = Float.sqrt (x **. 2. +. y **. 2.);;

Because the name of the fields (,*x*in the point2d type) and the name of the variable it is bound to (the argument input to magnitude) coincide, we don’t have to write them down.**y** - Using the
**not notation**:

12let distance v1 v2 =magnitude { x = v1.x -. v2.x; y = v1.y -. v2.y };;

Here,and**v1**are of type**v2**. We access the x field of v1 with the**point2d**notation, and the y field of v1 with the*v1.x*notation. Similarly for v2.**v1.y**

We can include our newly defined types as components in larger types. E.g.:

1 2 3 4 5 |
type circle_desc = { center: point2d; radius: float };; type rect_desc = { lower_left: point2d; width: float; height: float };; type segment_desc = { endpoint1: point2d; endpoint2: point2d } ;; |

### Variant

The type * variant* combine multiple objects of different types together as one type. E.g.:

1 2 3 4 5 |
type scene_element = | Circle of circle_desc | Rect of rect_desc | Segment of segment_desc ;; |

The * |* character separates the different cases of the variant (the first

**is optional), and each case has a capitalized tag, like**

*|***, to distinguish that case from the others.**

*Circle*I think I’ll stop here before I go into the next section: imperative programming.