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)
51The same names can be defined in multiple modules. This is possible
52because in practice names are *qualified* with the name of the module.
53The names defined in the ``btree`` module are, in full:
62If names are otherwise unambiguous, there is no need to give the fully
63qualified name. Names can be disambiguated either by giving an explicit
64qualification, or according to their type.
66There is no formal link between the module name and its filename,
67although it is generally advisable to use the same name for each. An
68``import`` statement refers to a filename, using dots to separate
69directories. For example, ``import foo.bar`` would import the file
70``foo/bar.idr``, which would conventionally have the module declaration
71``module foo.bar``. The only requirement for module names is that the
72main module, with the ``main`` function, must be called
73``Main``—although its filename need not be ``Main.idr``.
78By default, all names defined in a module are exported for use by other
79modules. However, it is good practice only to export a minimal interface
80and keep internal details abstract. Idris allows functions, types,
81and classes to be marked as: ``public``, ``abstract`` or ``private``:
83- ``public`` means that both the name and definition are exported. For
84 functions, this means that the implementation is exported (which
85 means, for example, it can be used in a dependent type). For data
86 types, this means that the type name and the constructors are
87 exported. For classes, this means that the class name and method
90- ``abstract`` means that only the name is exported. For functions,
91 this means that the implementation is not exported. For data types,
92 this means that the type name is exported but not the constructors.
93 For classes, this means that the class name is exported but not the
96- ``private`` means that neither the name nor the definition is
100 If any definition is given an export modifier, then all names with no modifier are assumed to be ``private``.
102For our ``btree`` module, it makes sense for the tree data type and the
103functions to be exported as ``abstract``, as we see below:
109 abstract data BTree a = Leaf
110 | Node (BTree a) a (BTree a)
113 insert : Ord a => a -> BTree a -> BTree a
114 insert x Leaf = Node Leaf x Leaf
115 insert x (Node l v r) = if (x < v) then (Node (insert x l) v r)
116 else (Node l v (insert x r))
119 toList : BTree a -> List a
121 toList (Node l v r) = btree.toList l ++ (v :: btree.toList r)
124 toTree : Ord a => List a -> BTree a
126 toTree (x :: xs) = insert x (toTree xs)
128Finally, the default export mode can be changed with the ``%access``
129directive, for example:
131In this case, any function with no access modifier will be exported as
132``abstract``, rather than left ``private``.
134Additionally, a module can re-export a module it has imported, by using
135the ``public`` modifier on an ``import``. For example:
144 public a : AType a = ...
146The module ``A`` will export the name ``a``, as well as any public or
147abstract names in module ``C``, but will not re-export anything from
153Defining a module also defines a namespace implicitly. However,
154namespaces can also be given *explicitly*. This is most useful if you
155wish to overload names within the same module:
166 test : String -> String
169This (admittedly contrived) module defines two functions with fully
170qualified names ``foo.x.test`` and ``foo.y.test``, which can be
171disambiguated by their types:
183Groups of functions can be parameterised over a number of arguments
184using a ``parameters`` declaration, for example:
188 parameters (x : Nat, y : Nat)
192The effect of a ``parameters`` block is to add the declared parameters
193to every function, type and data constructor within the block. Outside
194the block, the parameters must be given explicitly:
199 addAll : Nat -> Nat -> Nat -> Nat
201Parameters blocks can be nested, and can also include data declarations,
202in which case the parameters are added explicitly to all type and data
203constructors. They may also be dependent types with implicit arguments:
207 parameters (y : Nat, xs : Vect x a)
208 data Vects : Type -> Type where
209 MkVects : Vect y a -> Vects a
211 append : Vects a -> Vect (x + y) a
212 append (MkVects ys) = xs ++ ys
214To use ``Vects`` or ``append`` outside the block, we must also give the
215``xs`` and ``y`` arguments. Here, we can use placeholders for the values
216which can be inferred by the type checker:
220 *params> show (append _ _ (MkVects _ [1,2,3] [4,5,6]))
221 "[1, 2, 3, 4, 5, 6]" : String
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