5  Modules

A module defines a collection of values, datatypes, type synonyms, classes, etc. (see Section 4) in an environment created by a set of imports, resources brought into scope from other modules, and exports some of these resources, making them available to other modules. We use the term entity to refer to a value, type, or classes defined in, imported into, or perhaps exported from a module.

A Haskell program is a collection of modules, one of which, by convention, must be called Main and must export the value main. The value of the program is the value of the identifier main in module Main, and main must have type IO () (see Section 7).

Modules may reference other modules via explicit import declarations, each giving the name of a module to be imported and specifying its entities to be imported. Modules may be mutually recursive.

The name-space for modules is flat, with each module being associated with a unique module name (which are Haskell identifiers beginning with a capital letter; i.e. modid). There is one distinguished module, Prelude, which is imported into all programs by default (see Section 5.3), plus a set of standard library modules that may be imported as required (see the Haskell Library Report[8]).

5.1  Module Structure

A module defines a mutually recursive scope containing declarations for value bindings, data types, type synonyms, classes, etc. (see Section 4).

module	->	module modid [exports] where body
	|	body
body	->	{ [impdecls ;] [[fixdecls ;] topdecls [;]] }
	|	{ impdecls [;] }
modid	->	conid
impdecls	->	impdecl1 ; ... ; impdecln	(n>=1)
topdecls	->	topdecl1 ; ... ; topdecln	(n>=0)

A module begins with a header: the keyword module, the module name, and a list of entities (enclosed in round parentheses) to be exported. The header is followed by an optional list of import declarations that specify modules to be imported, optionally restricting the imported bindings. This is followed by an optional list of fixity declarations and the module body. The module body is simply a list of top-level declarations (topdecls), as described in Section 4.

An abbreviated form of module, consisting only of the module body, is permitted. If this is used, the header is assumed to be `module Main(main) where'. If the first lexeme in the abbreviated module is not a {, then the layout rule applies for the top level of the module.

5.1.1  Export Lists

exports	->	( export1 , ... , exportn [ , ] )	(n>=0)
export	->	qvar
	|	qtycon [(..) | ( qcname1 , ... , qcnamen )]	(n>=0)
	|	qtycls [(..) | ( qvar1 , ... , qvarn )]	(n>=0)
	|	module modid
qcname	->	qvar | qcon

An export list identifies the entities to be exported by a module declaration. A module implementation may only export an entity that it declares, or that it imports from some other module. If the export list is omitted, all values, types and classes defined in the module are exported, but not those that are imported.

Entities in an export list may be named as follows:

1. A value, field name, or class method, whether declared in the module body or imported, may be named by giving the name of the value as a qvarid. Operators should be enclosed in parentheses to turn them into qvarid's.
2. An algebraic datatype T declared by a data or newtype declaration may be named in one of three ways:
   • The form T names the type but not the constructors or field names. The ability to export a type without its constructors allows the construction of abstract datatypes (see Section 5.5).
   • The form T(qcname₁,...,qcname_n), where the qcname_i name only constructors and field names in T, names the type and some or all of its constructors and field names. The qcname_i must not contain duplications.
   • The abbreviated form T(..) names the type and all its constructors and field names that are currently in scope (whether qualified or not).
   Data constructors cannot be named in export lists in any other way.
3. A type synonym T declared by a type declaration may be named by the form T.
4. A class C with operations f₁,...,f_n declared in a class declaration may be named in one of three ways:
   • The form C names the class but not the class methods.
   • The form C(f₁,...,f_n), where the f_i must be class methods C, names the class and some or all of its methods. The f_i must not contain duplications.
   • The abbreviated form C(..) names the class and all its methods that are in scope (whether qualified or not).
5. The set of all entities brought into scope from a module m by one or more unqualified import declarations may be named by the form `module m', which is equivalent to listing all of the entities imported from the module. For example:
   
         module Queue( module Stack, enqueue, dequeue ) where
             import Stack
             ...
   
   Here the module Queue uses the module name Stack in its export list to abbreviate all the entities imported from Stack.

6. A module can name its own local definitions in its export list using its name in the `module m' syntax. For example:
   
          module Mod1(module Mod1, module Mod2) where
          import Mod2
          import Mod3
   
   Here module Mod1 exports all local definitions as well as those imported from Mod2 but not those imported from Mod3. Note that module M where is the same as using module M(module M) where.

5.1.2  Import Declarations

impdecl	->	import [qualified] modid [as modid] [impspec]
impspec	->	( import1 , ... , importn [ , ] )	(n>=0)
	|	hiding ( import1 , ... , importn [ , ] )	(n>=0)
import	->	var
	|	tycon [ (..) | ( cname1 , ... , cnamen )]	(n>=1)
	|	tycls [(..) | ( var1 , ... , varn )]	(n>=0)
cname	->	var | con

The entities exported by a module may be brought into scope in another module with an import declaration at the beginning of the module. The import declaration names the module to be imported and optionally specifies the entities to be imported. A single module may be imported by more than one import declaration. Imported names serve as top level declarations: they scope over the entire body of the module but may be shadowed by local non-top-level bindings. The effect of multiple import declarations is cumulative: an entity is in scope if it is named by any of the import declarations in a module. The ordering of imports is irrelevant.

Exactly which entities are to be imported can be specified in one of three ways:

1. The imported entities can be specified explicitly by listing them in parentheses. Items in the list have the same form as those in export lists, except qualifiers are not permitted and the `module modid' entity is not permitted. When the (..) form of import is used for a type or class, the (..) refers to all of the constructors, methods, or field names exported from the module.

   The list must name only entities exported by the imported module. The list may be empty, in which case nothing except the instances are imported.

2. Entities can be excluded by using the form hiding(import₁ , ... , import_n ), which specifies that all entities exported by the named module should imported except for those named in the list. Data constructors may be named directly in hiding lists without being prefixed by the associated type. Thus, in
   
   import M hiding (C)
   
   any constructor, class, or type named C is excluded. In contrast, using C in an import list names only a class or type. The hiding clause only applies to unqualified names. In the previous example, the name M.C is brought into scope. A hiding clause has no effect in an import qualified declaration.

   The effect of multiple import declarations is strictly cumulative: hiding an entity on one import declaration does not prevent the same entity from being imported by another import from the same module.

3. Finally, if impspec is omitted then all the entities exported by the specified module are imported.

When an import declaration uses the qualified keyword, the names brought into scope must be prefixed by the name of the imported module (or a local alias, if an as clause is present). A qualified name is written as modid.name. This allows full programmer control of the unqualified namespace: a locally defined entity can share the same name as a qualified import:

module Ring where
import qualified Prelude  -- All Prelude names must be qualified

l1 + l2 = l1 ++ l2        -- This + differs from the one in the Prelude
l1 * l2 = nub (l1 + l2)

succ = (Prelude.+ 1)

The qualifier does not change the syntactic treatment of a name: Prelude.+ is an infix operator with the same fixity as the definition of + in the Prelude. Qualifiers may be applied to names imported by an unqualified import; this allows a qualified import to be replaced with an unqualified one without forcing changes in the references to the imported names.

Imported modules may be assigned a local alias in the importing module using the as clause. For example, in

import qualified Complex as C

entities must be referenced using `C.' as a qualifier instead of `Complex.'. This also allows a different module to be substituted for Complex without changing the qualifiers used for the imported module. It is an error for more than one module in scope to use the same qualifier. Qualifiers can only be used for imported entities: locally defined names within a module may not include a qualifier unless the module explicitly imports itself.

Since qualifier names are part of the lexical syntax, no spaces are allowed between the qualifier and the name. Sample parses are shown below.

This	Lexes as this
f.g	f . g (three tokens)
F.g	F.g (qualified `g')
f..	f .. (two tokens)
F..	F.. (qualified `.')
F.	F . (two tokens)

It may be that a particular entity is imported into a module by more than one route --- for example, because it is exported by two modules, both of which are imported by a third module. Benign name clashes of this form are allowed, but it is a static error for two different entities to have the same name. When two entities have the same name, they are considered to be the same object if and only if they are defined by the same module. Two different qualified names may refer to the same entity; the name of the importing module does not affect the identity of an entity.

It is an error for two different entities to have the same name. This is valid:

module A
import B(f)
import qualified C(f)

as long as only one imported f is unqualified and f is not defined at the top level of A. Qualifiers are the only way to resolve name clashes between imported entities.

5.1.3  Importing and Exporting Instance Declarations

5.2  Closure

The type of an exported entity is unaffected by non-exported type synonyms. For example, in

module M(x) where
type T = Int
x :: T
x = 1

the type of x is both T and Int; these are interchangeable even when T is not in scope. That is, the definition of T is available to any module that encounters it whether or not the name T is in scope. The only reason to export T is to allow other modules to refer it by name; the type checker finds the definition of T if needed whether or not it is exported.

5.3  Standard Prelude

Prelude and library modules differ from other modules in that their semantics (but not their implementation) are a fixed part of the Haskell language definition. This means, for example, that a compiler may optimize calls to functions in the Prelude without being concerned that a future change to the program will alter the semantics of the Prelude function.

5.3.1  The Prelude Module

The Prelude module is imported automatically into all modules as if by the statement `import Prelude', if and only if it is not imported with an explicit import declaration. This provision for explicit import allows values defined in the Prelude to be hidden from the unqualified name space. The Prelude module is always available as a qualified import: an implicit `import qualified Prelude' is part of every module and names prefixed by `Prelude.' can always be used to refer to entities in the Prelude.

The semantics of the entities in Prelude is specified by an implementation of Prelude written in Haskell , given in Appendix A. Some datatypes (such as Int) and functions (such as Int addition) cannot be specified directly in Haskell . Since the treatment of such entities depends on the implementation, they are not formally defined in the appendix. The implementation of Prelude is also incomplete in its treatment of tuples: there should be an infinite family of tuples and their instance declarations, but the implementation only gives a scheme.

5.3.2  Shadowing Prelude Names

The rules about the Prelude have been cast so that it is possible to use Prelude names for nonstandard purposes; however, every module that does so must have an import declaration that makes this nonstandard usage explicit. For example:

     module A where
     import Prelude hiding (null)
     null x = []

Module A redefines null, but it must indicate this by importing Prelude without null. Furthermore, A exports null, but every module that imports null unqualified from A must also hide null from Prelude just as A does. Thus there is little danger of accidentally shadowing Prelude names.

It is possible to construct and use a different module to serve in place of the Prelude. Other than the fact that it is implicitly imported, the Prelude is an ordinary Haskell module; it is special only in that some objects in the Prelude are referenced by special syntactic constructs. Redefining names used by the Prelude does not affect the meaning of these special constructs. For example, in

module B where
import qualified Prelude
import MyPrelude
      ...

B imports nothing from Prelude, but the explicit import qualified Prelude declaration prevents the automatic import of Prelude. import MyPrelude brings the non-standard prelude into scope. As before, the standard prelude names are hidden explicitly. Special syntax, such as lists or tuples, always refers to prelude entities: there is no way to redefine the meaning of [x] in terms of a different implementation of lists.

5.4  Separate Compilation

5.5  Abstract Datatypes

The ability to export a datatype without its constructors allows the construction of abstract datatypes (ADTs). For example, an ADT for stacks could be defined as:

module Stack( StkType, push, pop, empty ) where
data StkType a = EmptyStk | Stk a (StkType a)
push x s = Stk x s
pop (Stk _ s) = s
empty = EmptyStk

Modules importing Stack cannot construct values of type StkType because they do not have access to the constructors of the type.

It is also possible to build an ADT on top of an existing type by using a newtype declaration. For example, stacks can be defined with lists:

module Stack( StkType, push, pop, empty ) where
newtype StkType a = Stk [a]
push x (Stk s) = Stk (x:s)
pop (Stk (x:s)) = Stk s
empty = Stk []

5.6  Fixity Declarations

fixdecls	->	fix1 ; ... ; fixn	(n>=1)
fix	->	infixl [digit] ops
	|	infixr [digit] ops
	|	infix  [digit] ops
ops	->	op1 , ... , opn	(n>=1)
op	->	varop | conop

A fixity declaration gives the fixity and binding precedence of a set of operators. Fixity declarations must appear only at the start of a module and may only be given for identifiers defined in that module. Fixity declarations cannot subsequently be overridden, and an identifier can only have one fixity definition.

There are three kinds of fixity, non-, left- and right-associativity (infix, infixl, and infixr, respectively), and ten precedence levels, 0 to 9 inclusive (level 0 binds least tightly, and level 9 binds most tightly). If the digit is omitted, level 9 is assumed. Any operator lacking a fixity declaration is assumed to be infixl 9 (See Section 3 for more on the use of fixities). Table 2 lists the fixities and precedences of the operators defined in the Prelude.

Fixity is a property of the name of an identifier or operator: the same fixity attaches to every occurrence of an operator name in a module, whether at the top level or rebound at an inner level. For example:

module Foo
import Bar
infix 3 `op`

f x = ... where p `op` q = ...

Here `op` has fixity 3 wherever it is in scope, provided Bar does not export the identifier op. If Bar does export op, then the example becomes invalid, because the fixity (or lack thereof) of op is defined in Bar (or wherever Bar imported op from). If op is imported as a qualified name from Bar, no conflict may occur: the fixity of a qualified name does not affect unqualified uses of the same name.