Modules
Table of Contents |
Basics
First let us introduce some terminology. A definition is a syntactic construction defining an entity such as a function or a datatype. A name is a string used to identify definitions. The same definition can have many names and at different points in the program it will have different names. It may also be the case that two definitions have the same name. In this case there will be an error if the name is used.
The main purpose of the module system is to structure the way names are used in a program. This is done by organising the program in an hierarchical structure of modules where each module contains a number of definitions and submodules. For instance,
module Main where data Nat : Set where zero : Nat suc : Nat → Nat module B where f : Nat → Nat f n = suc n g : Nat → Nat → Nat g n m = m
Note that we use indentation to indicate which definitions are part of a module.
In the example f
is in the module Main.B
and g
is in Main
.
How to refer to a particular definition is determined by where it is located in
the module hierarchy. Definitions from an enclosing module are referred to by
their given names as seen in the type of f
above. To access a definition from
outside its defining module a qualified name has to be used.
module Main where data Nat : Set where zero : Nat suc : Nat → Nat module B where f : Nat → Nat f n = suc n ff : Nat → Nat ff x = B.f (B.f x)
To be able to use the short names for definitions in a module the module has to be opened.
module Main where data Nat : Set where zero : Nat suc : Nat → Nat module B where f : Nat → Nat f n = suc n open B ff : Nat → Nat ff x = f (f x)
If A.qname
refers to a definition d
then after open A
, qname
will also
refer to d
. Note that qname
can itself be a qualified name. Opening a module
only introduces new names for a definition, it never removes the old names. The
policy is to allow the introduction of ambiguous names, but give an error if an
ambiguous name is used.
Modules can also be opened within a local scope by putting the open B
within a where
clause:
ff : Nat → Nat ff x = f (f x) where open B
Private definitions
To make a definition inaccessible outside its defining module it can be declared
private
. A private definition is treated as a normal definition inside the
module that defines it, but outside the module the definition has no name. In a
dependently type setting there are some problems with private definitions---since
the type checker performs computations, private names might show up in goals and
error messages. Consider the following (contrived) example
module Main where module A where private IsZero’ : Nat → Set IsZero’ Zero = True IsZero’ (Suc n) = False IsZero : Nat → Set IsZero n = IsZero’ n open A prf : (n : Nat) → IsZero n prf n = ?_{0}
The type of the goal ?_{0}
is IsZero n
which normalises to IsZero’ n
. The
question is how to display this normal form to the user. At the point of ?_{0}
there is no name for IsZero’
. One option could be try to fold the term and
print IsZero n
. This is a very hard problem in general, so rather than
trying to do this we make it clear to the user that IsZero’
is something
that is not in scope and print the goal as .Main.A.IsZero’ n
. The leading
dot indicates that the entity is not in scope. The same technique is used for
definitions that only have ambiguous names.
In effect using private definitions means that from the user’s perspective we do not have subject reduction. This is just an illusion, however---the type checker has full access to all definitions.
Name modifiers
An alternative to making definitions private is to exert finer control over what
names are introduced when opening a module. This is done by qualifying an open
statement with one or more of the modifiers using
, hiding
, or renaming
. You
can combine both using
and hiding
with renaming
, but not with each other. The
effect of
open A using (xs) renaming (ys to zs)
is to introduce the names xs
and zs
where xs
refers to the same
definition as A.xs
and zs
refers to A.ys
. Note that if xs
and
ys
overlap there will be two names introduced for the same definition. We
do not permit xs
and zs
to overlap. The other forms of opening are defined
in terms of this one. Let A
denote all the (public) names in A
. Then
open A renaming (ys to zs) == open A hiding () renaming (ys to zs) open A hiding (xs) renaming (ys to zs) == open A using (A - xs - ys) renaming (ys to zs)
An omitted renaming
modifier is equivalent to an empty renaming.
Re-exporting names
A useful feature is the ability to re-export names from another module. For
instance, one may want to create a module to collect the definitions from
several other modules. This is achieved by qualifying the open statement with
the public
keyword:
module Nat where Nat : Set module Bool where Bool : Set module Prelude where open Nat public open Bool public isZero : Nat → Bool
This will introduce definitions Prelude.Nat
and Prelude.Bool
which are
visible from outside the Prelude
module.
Parameterised modules
So far, the module system features discussed have dealt solely with scope manipulation. We now turn our attention to some more advanced features.
It is sometimes useful to be able to work temporarily in a given signature. For
instance, when defining functions for sorting lists it is convenient to assume a
set of list elements A
and an ordering over A
. In Coq
this can be done in two ways: using a functor, which is essentially a
function between modules, or using a section. A section allows you to
abstract some arguments from several definitions at once. We introduce
parameterised modules analogous to sections in Coq. When declaring a module you
can give a telescope of module parameters which are abstracted from all the
definitions in the module.
For instance, a simple implementation of a sorting function looks like this:
module Sort (A : Set)(_≤_ : A → A → Bool) where insert : A → List A → List A insert x ε = x :: ε insert x (y :: ys) with x ≤ y insert x (y :: ys) | true = x :: y :: ys insert x (y :: ys) | false = y :: insert x ys sort : List A → List A sort ε = ε sort (x :: xs) = insert x (sort xs)
As mentioned parametrising a module has the effect of abstracting the
parameters over the definitions in the module, so outside the Sort
module we
have
Sort.insert : (A : Set)(_≤_ : A → A → Bool) → A → List A → List A Sort.sort : (A : Set)(_≤_ : A → A → Bool) → List A → List A
For function definitions, explicit module parameter become explicit arguments to the abstracted function, and implicit parameters become implicit arguments. For constructors, however, the parameters are always implicit arguments. This is a consequence of the fact that module parameters are turned into datatype parameters, and the datatype parameters are implicit arguments to the constructors. It also happens to be the reasonable thing to do.
Something which you cannot do in Coq is to apply a section to its arguments. We allow this through the module application statement. In our example:
module SortNat = Sort Nat leqNat
This will define a new module SortNat
as follows
module SortNat where insert : Nat → List Nat → List Nat insert = Sort.insert Nat leqNat sort : List Nat → List Nat sort = Sort.sort Nat leqNat
The new module can also be parameterised, and you can use name modifiers to control what definitions from the original module are applied and what names they have in the new module. The general form of a module application is
module M_{1} Δ = M_{2} terms modifiers
A common pattern is to apply a module to its arguments and then open the resulting module. To simplify this we introduce the short-hand
open module M_{1} Δ = M_{2} terms [public] mods
for
module M_{1} Δ = M_{2} terms mods open M_{1} [public]
Splitting a program over multiple files
When building large programs it is crucial to be able to split the program over
multiple files and to not have to type check and compile all the files for every
change. The module system offers a structured way to do this. We
define a program to be a collection of modules, each module being defined in a
separate file. To gain access to a module defined in a different file you can
import
the module:
import M
In order to implement this we must be able to find the file in which a module is
defined. To do this we require that the top-level module A.B.C
is defined in
the file C.agda
in the directory A/B/
. One could imagine instead to
give a file name to the import statement, but this would mean cluttering the
program with details about the file system which is not very nice.
When importing a module M
the module and its contents is brought into scope as
if the module had been defined in the current file. In order to get access to
the unqualified names of the module contents it has to be opened. Similarly to
module application we introduce the short-hand
open import M
for
import M open M
Sometimes the name of an imported module clashes with a local module. In this case it is possible to import the module under a different name.
import M as M’
It is also possible to attach modifiers to import statements, limiting or changing what names are visible from inside the module.
Datatype modules
When you define a datatype it also defines a module so constructors can now be referred to qualified by their data type.
For instance, given
data Nat : Set where zero : Nat suc : Nat → Nat data Fin : Nat → Set where zero : ∀ {n} → Fin (suc n) suc : ∀ {n} → Fin n → Fin (suc n)
you can refer to the constructors unambiguously as Nat.zero, Nat.suc, Fin.zero, and Fin.suc (Nat and Fin are modules containing the respective constructors). Example:
inj : (n m : Nat) → Nat.suc n ≡ suc m → n ≡ m inj .m m refl = refl
Previously you had to write something like
inj : (n m : Nat) → _≡_ {Nat} (suc n) (suc m) → n ≡ m
to make the type checker able to figure out that you wanted the natural number suc in this case.
Record update syntax.
Assume that we have a record type and a corresponding value:
record MyRecord : Set where field a b c : ℕ old : MyRecord old = record { a = 1; b = 2; c = 3 }
Then we can update (some of) the record value’s fields in the following way:
new : MyRecord new = record old { a = 0; c = 5 }
Here new normalises to record { a = 0; b = 2; c = 5 }. Any expression yielding a value of type MyRecord can be used instead of old.
Record updating is not allowed to change types: the resulting value must have the same type as the original one, including the record parameters. Thus, the type of a record update can be inferred if the type of the original record can be inferred.
The record update syntax is expanded before type checking. When the expression
record old { upd-fields }
is checked against a record type R, it is expanded to
let r = old in record { new-fields },
where old is required to have type R and new-fields is defined as follows: for each field x in R,
- if x = e is contained in upd-fields then x = e is included in new-fields, and otherwise - if x is an explicit field then x = R.x r is included in new-fields, and - if x is an implicit or instance field, then it is omitted from new-fields.
(Instance arguments are explained below.) The reason for treating implicit and instance fields specially is to allow code like the following:
record R : Set where field {length} : ℕ vec : Vec ℕ length — More fields… xs : R xs = record { vec = 0 ∷ 1 ∷ 2 ∷ [] } ys = record xs { vec = 0 ∷ [] }
Without the special treatment the last expression would need to include a new binding for length (for instance “length = _”).