7An Idris program consists of a collection of modules. Each module
8includes an optional ``module`` declaration giving the name of the
9module, a list of ``import`` statements giving the other modules which
10are to be imported, and a collection of declarations and definitions of
11types, classes and functions. For example, the listing below gives a
12module which defines a binary tree type ``BTree`` (in a file
20 | Node (BTree a) a (BTree a)
22 insert : Ord a => a -> BTree a -> BTree a
23 insert x Leaf = Node Leaf x Leaf
24 insert x (Node l v r) = if (x < v) then (Node (insert x l) v r)
25 else (Node l v (insert x r))
27 toList : BTree a -> List a
29 toList (Node l v r) = btree.toList l ++ (v :: btree.toList r)
31 toTree : Ord a => List a -> BTree a
33 toTree (x :: xs) = insert x (toTree xs)
36Then, this gives a main program (in a file
37``bmain.idr``) which uses the ``bst`` module to sort a list:
46 main = do let t = toTree [1,8,2,7,9,3]
47 print (btree.toList t)
50The same names can be defined in multiple modules. This is possible
51because in practice names are *qualified* with the name of the module.
52The names defined in the ``btree`` module are, in full:
61If names are otherwise unambiguous, there is no need to give the fully
62qualified name. Names can be disambiguated either by giving an explicit
63qualification, or according to their type.
65There is no formal link between the module name and its filename,
66although it is generally advisable to use the same name for each. An
67``import`` statement refers to a filename, using dots to separate
68directories. For example, ``import foo.bar`` would import the file
69``foo/bar.idr``, which would conventionally have the module declaration
70``module foo.bar``. The only requirement for module names is that the
71main module, with the ``main`` function, must be called
72``Main``—although its filename need not be ``Main.idr``.
77By default, all names defined in a module are exported for use by other
78modules. However, it is good practice only to export a minimal interface
79and keep internal details abstract. Idris allows functions, types,
80and classes to be marked as: ``public``, ``abstract`` or ``private``:
82- ``public`` means that both the name and definition are exported. For
83 functions, this means that the implementation is exported (which
84 means, for example, it can be used in a dependent type). For data
85 types, this means that the type name and the constructors are
86 exported. For classes, this means that the class name and method
89- ``abstract`` means that only the name is exported. For functions,
90 this means that the implementation is not exported. For data types,
91 this means that the type name is exported but not the constructors.
92 For classes, this means that the class name is exported but not the
95- ``private`` means that neither the name nor the definition is
99 If any definition is given an export modifier, then all names with no modifier are assumed to be ``private``.
101For our ``btree`` module, it makes sense for the tree data type and the
102functions to be exported as ``abstract``, as we see below:
108 abstract data BTree a = Leaf
109 | Node (BTree a) a (BTree a)
112 insert : Ord a => a -> BTree a -> BTree a
113 insert x Leaf = Node Leaf x Leaf
114 insert x (Node l v r) = if (x < v) then (Node (insert x l) v r)
115 else (Node l v (insert x r))
118 toList : BTree a -> List a
120 toList (Node l v r) = btree.toList l ++ (v :: btree.toList r)
123 toTree : Ord a => List a -> BTree a
125 toTree (x :: xs) = insert x (toTree xs)
127Finally, the default export mode can be changed with the ``%access``
128directive, for example:
130In this case, any function with no access modifier will be exported as
131``abstract``, rather than left ``private``.
133Additionally, a module can re-export a module it has imported, by using
134the ``public`` modifier on an ``import``. For example:
140 import B import public C
142 public a : AType a = ...
144The module ``A`` will export the name ``a``, as well as any public or
145abstract names in module ``C``, but will not re-export anything from
151Defining a module also defines a namespace implicitly. However,
152namespaces can also be given *explicitly*. This is most useful if you
153wish to overload names within the same module:
164 test : String -> String
167This (admittedly contrived) module defines two functions with fully
168qualified names ``foo.x.test`` and ``foo.y.test``, which can be
169disambiguated by their types:
181Groups of functions can be parameterised over a number of arguments
182using a ``parameters`` declaration, for example:
186 parameters (x : Nat, y : Nat)
190The effect of a ``parameters`` block is to add the declared parameters
191to every function, type and data constructor within the block. Outside
192the block, the parameters must be given explicitly:
197 addAll : Nat -> Nat -> Nat -> Nat
199Parameters blocks can be nested, and can also include data declarations,
200in which case the parameters are added explicitly to all type and data
201constructors. They may also be dependent types with implicit arguments:
205 parameters (y : Nat, xs : Vect x a)
206 data Vects : Type -> Type where
207 MkVects : Vect y a -> Vects a
209 append : Vects a -> Vect (x + y) a
210 append (MkVects ys) = xs ++ ys
212To use ``Vects`` or ``append`` outside the block, we must also give the
213``xs`` and ``y`` arguments. Here, we can use placeholders for the values
214which can be inferred by the type checker:
218 *params> show (append _ _ (MkVects _ [1,2,3] [4,5,6]))
219 "[1, 2, 3, 4, 5, 6]" : String
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