Modules

Modules are a programming language construction that allows us to package related definitions together. A canonical example of a module is a data type and associated operations over it (e.g. stacks or queues). The rest of the program can access these definitions in a regular and abstract way, providing maintainability, reusability and safety.

For a concrete example, we could create a module that packages a type that represents amounts in a particular currency together with functions that manipulate these amounts: constants, addition, subtraction, etc. A piece of code that uses this module can be agnostic concerning how the type is actually represented inside the module: it is said to be abstract.

Declaring Modules

Modules are introduced using the module keyword. For example, the following code defines a module EURO that packages together a type, called t, together with an operation add that sums two values of the given currency, as well as constants for zero and one.

module EURO =

struct

type t = nat

let add (a , b : t * t) : t = a + b

let zero : t = 0n

let one : t = 1n

end

As we can see, in CameLIGO we also use a struct ... end block to group together the definitions made in the module.

Using Modules

We can access a module's components by using the selection operator .. Let us suppose that our storage keeps a value in euros using the previously defined module EURO. Then, we can write a main entry point that increments the storage value each time it is called.

type storage = EURO.t

[@entry]

let main (_action : unit) (store : storage) : operation list * storage =

[], EURO.add (store, EURO.one)

In principle, we could change the implementation of EURO, without having to change the storage type or the function main. For example, if we decide later that we should support manipulating negative values, we could change EURO as follows:

module EURO =

struct

type t = int

let add (a, b : t * t) : t = a + b

let zero : t = 0

let one : t = 1

end

Notice that the code in main still works, and no change is needed. Abstraction accomplished!

⚠️ Please note that code using the module EURO might still break the abstraction if it directly uses the underlying representation of EURO.t. Client code should always try to respect the interface provided by the module, and not make assumptions on its current underlying representation (e.g. EURO.t is a transparent alias of nat; future versons of LIGO might make this an opaque/abstract type).

Nested Modules: Sub-Modules

Modules can be nested, which means that we can define a module inside another module. Let's see how that works, and define a variant of EURO in which the constants are all grouped inside using a sub-module.

module EURO =

struct

type t = nat

let add (a, b : t * t) : t = a + b

module CONST =

struct

let zero : t = 0n

let one : t = 1n

end

end

To access nested modules we simply apply the selection operator more than once:

type storage = EURO.t

[@entry]

let main (_action : unit) (store : storage) : operation list * storage =

[], EURO.add (store, EURO.CONST.one)

Modules and Imports: Build System

Modules also allow us to separate our code in different files: when we import a file, we obtain a module encapsulating all the definitions in it. This will become very handy for organising large contracts, as we can divide it into different files, and the module system keeps the naming space clean.

Generally, we will take a set of definitions that can be naturally grouped by functionality, and put them together in a separate file.

For example, in CameLIGO, we can create a file imported.mligo:

type t = nat

let add (a , b : t * t) : t = a + b

let zero : t = 0n

let one : t = 1n

Later, in another file, we can import imported.mligo as a module, and use its definitions. For example, we could create a importer.mligo that imports all definitions from imported.mligo as the module EURO:

#import "gitlab-pages/docs/language-basics/src/modules/imported.mligo" "EURO"

type storage = EURO.t

[@entry]

let main (_action : unit) (store : storage) : operation list * storage =

([], EURO.add(store, EURO.one))

We can compile the file that uses the #import statement directly, without having to mention the imported file.

ligo compile contract --library . gitlab-pages/docs/language-basics/src/modules/importer.mligo

Module Aliases

LIGO supports module aliases, that is, modules that work as synonyms to other (previously defined) modules. This feature can be useful if we could implement a module using a previously defined one, but in the future, we might need to change it.

module US_DOLLAR = EURO

Modules as Contracts

When a module contains declarations that are tagged with the attribute @entry, then a contract can be obtained from such module. All declarations in the module tagged as @entry are grouped, and a dispatcher contract is generated.

module C = struct

[@entry] let increment (p : int) (s : int) : operation list * int = [], s + p

[@entry] let decrement (p : int) (s : int) : operation list * int = [], s - p

end

A module can be compiled as a contract using -m:

ligo compile contract gitlab-pages/docs/language-basics/src/modules/contract.mligo -m C

To access the contract from the module, the primitive contract_of can be used. The type of the parameter generated for the module can be obtaining using the primitive parameter_of. This is particularly useful when working with the testing framework, in conjunction with the function Test.originate:

let test =

let orig = Test.originate (contract_of C) 0 0mav in

let _ = Test.transfer_exn orig.addr (Increment 42) 0mav

in assert (42 = Test.get_storage orig.addr)

Module Inclusion

When writing a new version of a given contract derived from a module, it is often needed to add new features, that is, new types and values, for example when implementing the next version of a standard. This can be achieved by defining a new module that includes the types and values of the old one, and defines new ones.

The inclusion of a module M is specified with a field include M, like so:

module FA0 = struct

type t = unit

[@entry] let transfer (_ : unit) (_ : t) : operation list * t = [], ()

end

module FA0Ext = struct

include FA0

[@entry] let transfer2 (a : unit) (b : t) = transfer a b

end

Module Types

Until now, we dealt with implicit module types, also know as signatures. Having explicitly declared module types enable abstraction and reusability by inclusion of signatures. Module types are defined like in OCaml:

module type FA0_SIG = sig

type t

[@entry] val transfer : unit -> t -> operation list * t

end

module type FA0Ext_SIG = sig

include FA0_SIG

[@entry] val transfer2 : unit -> t -> operation list * t

end

Notice how t in the type of transfer2 refers to t in the signature FA0_SIG and remains abstract. We can now revisit the examples above by constraining the module definitions with the module types:

module FA0 : FA0_SIG = struct

type t = unit

[@entry] let transfer (_ : unit) (_ : t) : operation list * t = [], ()

end

module FA0Ext : FA0Ext_SIG = struct

include FA0

[@entry] let transfer2 (a : unit) (b : t) = transfer a b

end

Note how module definitions must instantiate any abstract type (here FA0Impl.t). Also, when a module is constrained by a signature, it must implement the types and values in the latter, but no more: this is a filtering semantics.