Optionals are a pervasive programing pattern in OCaml. Since Michelson and LIGO are both inspired by OCaml, optional types are available in LIGO as well. Similarly, OCaml features a unit type, and LIGO features it as well. Both the option type and the unit types are instances of a more general kind of types: variant types (sometimes called sum types).
Unit
The unit type in Michelson or LIGO is a predefined type that contains only one value that carries no information. It is used when no relevant information is required or produced. Here is how it used.
In CameLIGO, the unique value of the unit type is (), following the OCaml convention.
let n : unit = ()
Variant
A variant type is a user-defined or a built-in type (in case of options) that defines a type by cases, so a value of a variant type is either this, or that or… The simplest variant type is equivalent to the enumerated types found in Java, C++, JavaScript etc.
Here is how we define a coin as being either head or tail (and nothing else):
type coin = Head | Tail let head : coin = Head let tail : coin = Tail
The names Head and Tail in the definition of the type coin are called data constructors, or variants. In this particular, they carry no information beyond their names, so they are called constant constructors.
In general, it is interesting for variants to carry some information, and thus go beyond enumerated types. In the following, we show how to define different kinds of users of a system.
type id = nat type user = Admin of id | Manager of id | Guest let u : user = Admin 1000n let g : user = Guest
In LIGO, a constant constructor is equivalent to the same constructor taking an argument of type unit, so, for example, Guest is the same value as Guest (unit).
Pattern matching
Pattern matching is similar to the switch construct in Javascript, and can be used to route the program’s control flow based on the value of a variant. Consider for example the definition of a function flip that flips a coin.
type coin = Head | Tail
let flip (c : coin) : coin =
match c with
Head -> Tail
| Tail -> Head
Notice the switch construct is using the match with instruction to discriminate all possible values of c
Your mission
1- Create the type parameter as a variant.
2- Define the first option of the variant as Set_ship_code which will be our action to set the ship code. Notice this will take a string as an input.
3- Define the second option of the variant as Go_to which will be our action to set the ship on course to a new destination. Notice this will also take a string as an input.