The Haskell 98 Report
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5  Modules

A module defines a collection of values, datatypes, type synonyms, classes, etc. (see Chapter 4), in an environment created by a set of imports (resources brought into scope from other modules). It exports some of these resources, making them available to other modules. We use the term entity to refer to a value, type, or class 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, which must be a computation of type IO t for some type t (see Chapter 7). When the program is executed, the computation main is performed, and its result (of type t) is discarded.

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.

Modules are used for name-space control, and are not first class values. A multi-module Haskell program can be converted into a single-module program by giving each entity a unique name, changing all occurrences to refer to the appropriate unique name, and then concatenating all the module bodies (There are two minor exceptions to this statement. First, default declarations scope over a single module (Section 4.3.4). Second, Rule 2 of the monomorphism restriction (Section 4.5.5) is affected by module boundaries. ). For example, here is a three-module program:

  module Main where
    import A
    import B
    main = A.f >> B.f

  module A where
    f = ...

  module B where
    f = ...

It is equivalent to the following single-module program:

  module Main where
    main = af >> bf

    af = ...

    bf = ...

Because they are allowed to be mutually recursive, modules allow a program to be partitioned freely without regard to dependencies.

The name-space for modules themselves 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 modules by default (see Section 5.6), plus a set of standard library modules that may be imported as required (see Part II).

5.1  Module Structure

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

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

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 a possibly-empty list of import declarations (impdecls, Section 5.3) that specify modules to be imported, optionally restricting the imported bindings. This is followed by a possibly-empty list of top-level declarations (topdecls, Chapter 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.

5.2  Export Lists

exports -> ( export1 , ... , exportn [ , ] ) (n>=0)
export -> qvar
| qtycon [(..) | ( cname1 , ... , cnamen )] (n>=0)
| qtycls [(..) | ( var1 , ... , varn )] (n>=0)
| module modid
cname -> var | con

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, which must be in scope. Operators should be enclosed in parentheses to turn them into qvarids.

  2. An algebraic datatype T declared by a data or newtype declaration may be named in one of three ways: In all cases, the (possibly-qualified) type constructor T must be in scope. The constructor and field names ci in the second form are unqualified; one of these subordinate names is legal if and only if (a) it names a constructor or field of T, and (b) the constructor or field is in scope in the module body regardless of whether it is in scope under a qualified or unqualified name. For example, the following is legal

      module A( Mb.Maybe( Nothing, Just ) ) where
        import qualified Maybe as Mb

    Data constructors cannot be named in export lists except as subordinate names, because they cannot otherwise be distinguished from type constructors.

  3. A type synonym T declared by a type declaration may be named by the form T, where T is in scope.

  4. A class C with operations f1,...,fn declared in a class declaration may be named in one of three ways: In all cases, C must be in scope. In the second form, one of the (unqualified) subordinate names fi is legal if and only if (a) it names a class method of C, and (b) the class method is in scope in the module body regardless of whether it is in scope under a qualified or unqualified name.

  5. The form "module M" names the set of all entities that are in scope with both an unqualified name "e" and a qualified name "M.e". This set may be empty. 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.

    A module can name its own local definitions in its export list using its own name in the "module M" syntax, because a local declaration brings into scope both a qualified and unqualified name (Section 5.5.1). 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.

    It is an error to use module M in an export list unless M is the module bearing the export list, or M is imported by at least one import declaration (qualified or unqualified).

Exports lists are cumulative: the set of entities exported by an export list is the union of the entities exported by the individual items of the list.

It makes no difference to an importing module how an entity was exported. For example, a field name f from data type T may be exported individually (f, item (1) above); or as an explicitly-named member of its data type (T(f), item (2)); or as an implicitly-named member (T(..), item(2)); or by exporting an entire module (module M, item (5)).

The unqualified names of the entities exported by a module must all be distinct (within their respective namespace). For example

  module A ( C.f, C.g, g, module B ) where   -- an invalid module
  import B(f)
  import qualified C(f,g)
  g = f True

There are no name clashes within module A itself, but there are name clashes in the export list between C.g and g (assuming C.g and g are different entities -- remember, modules can import each other recursively), and between module B and C.f (assuming B.f and C.f are different entities).

5.3  Import Declarations

impdecl -> import [qualified] modid [as modid] [impspec]
| (empty declaration)
impspec -> ( import1 , ... , importn [ , ] ) (n>=0)
| hiding ( import1 , ... , importn [ , ] ) (n>=0)
import -> var
| tycon [ (..) | ( cname1 , ... , cnamen )] (n>=0)
| 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 strictly cumulative: an entity is in scope if it is imported by any of the import declarations in a module. The ordering of import declarations is irrelevant.

Lexically, the terminal symbols "as", "qualified" and "hiding" are each a varid rather than a reservedid. They have special significance only in the context of an import declaration; they may also be used as variables.

5.3.1  What is imported

Exactly which entities are to be imported can be specified in one of the following 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 is imported.

  2. Entities can be excluded by using the form hiding(import1 , ... , importn ), which specifies that all entities exported by the named module should be 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.

    It is an error to hide an entity that is not, in fact, exported by the imported module.

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

5.3.2  Qualified import

For each entity imported under the rules of Section 5.3.1, the top-level environment is extended. If the import declaration used the qualified keyword, only the qualified name of the entity is brought into scope. If the qualified keyword is omitted, then both the qualified and unqualified name of the entity is brought into scope. Section 5.5.1 describes qualified names in more detail.

The qualifier on the imported name is either the name of the imported module, or the local alias given in the as clause (Section 5.3.3) on the import statement. Hence, the qualifier is not necessarily the name of the module in which the entity was originally declared.

The ability to exclude the unqualified names 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
  import List( nub )

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

  succ = (Prelude.+ 1)

5.3.3  Local aliases

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

  import qualified VeryLongModuleName as C

entities must be referenced using `C.' as a qualifier instead of `VeryLongModuleName.'. This also allows a different module to be substituted for VeryLongModuleName without changing the qualifiers used for the imported module. It is legal for more than one module in scope to use the same qualifier, provided that all names can still be resolved unambiguously. For example:

  module M where
    import qualified Foo as A
    import qualified Baz as A
    x = A.f

This module is legal provided only that Foo and Baz do not both export f.

An as clause may also be used on an un-qualified import statement:

  import Foo as A(f)

This declaration brings into scope f and A.f.

5.3.4  Examples

To clarify the above import rules, suppose the module A exports x and y. Then this table shows what names are brought into scope by the specified import statement:

Import declaration Names brought into scope
import A x, y, A.x, A.y
import A() (nothing)
import A(x) x, A.x
import qualified A A.x, A.y
import qualified A() (nothing)
import qualified A(x) A.x
import A hiding () x, y, A.x, A.y
import A hiding (x) y, A.y
import qualified A hiding () A.x, A.y
import qualified A hiding (x) A.y
import A as B x, y, B.x, B.y
import A as B(x) x, B.x
import qualified A as B B.x, B.y
In all cases, all instance declarations in scope in module A are imported (Section 5.4).

5.4  Importing and Exporting Instance Declarations

Instance declarations cannot be explicitly named on import or export lists. All instances in scope within a module are always exported and any import brings all instances in from the imported module. Thus, an instance declaration is in scope if and only if a chain of import declarations leads to the module containing the instance declaration.

For example, import M() does not bring any new names in scope from module M, but does bring in any instances visible in M. A module whose only purpose is to provide instance declarations can have an empty export list. For example

  module MyInstances() where
    instance Show (a -> b) where
show fn = "<<function>>"
    instance Show (IO a) where
show io = "<<IO action>>"

5.5  Name Clashes and Closure

5.5.1  Qualified names

A qualified name is written as (Section 2.4). A qualified name is brought into scope:

5.5.2  Name clashes

If a module contains a bound occurrence of a name, such as f or A.f, it must be possible unambiguously to resolve which entity is thereby referred to; that is, there must be only one binding for f or A.f respectively.

It is not an error for there to exist names that cannot be so resolved, provided that the program does not mention those names. For example:

  module A where
    import B
    import C
    tup = (b, c, d, x)
  module B( d, b, x, y ) where
    import D
    x = ...
    y = ...
    b = ...
  module C( d, c, x, y ) where
    import D
    x = ...
    y = ...
    c = ...

  module D( d ) where
    d = ...

Consider the definition of tup.

The name occurring in a type signature or fixity declarations is always unqualified, and unambiguously refers to another declaration in the same declaration list (except that the fixity declaration for a class method can occur at top level --- Section 4.4.2). For example, the following module is legal:

  module F where

    sin :: Float -> Float
    sin x = (x::Float)

    f x = Prelude.sin (F.sin x)

The local declaration for sin is legal, even though the Prelude function sin is implicitly in scope. The references to Prelude.sin and F.sin must both be qualified to make it unambiguous which sin is meant. However, the unqualified name sin in the type signature in the first line of F unambiguously refers to the local declaration for sin.

5.5.3  Closure

Every module in a Haskell program must be closed. That is, every name explicitly mentioned by the source code must be either defined locally or imported from another module. However, entities that the compiler requires for type checking or other compile time analysis need not be imported if they are not mentioned by name. The Haskell compilation system is responsible for finding any information needed for compilation without the help of the programmer. That is, the import of a variable x does not require that the datatypes and classes in the signature of x be brought into the module along with x unless these entities are referenced by name in the user program. The Haskell system silently imports any information that must accompany an entity for type checking or any other purposes. Such entities need not even be explicitly exported: the following program is valid even though T does not escape M1:

  module M1(x) where
    data T = T
    x = T
  module M2 where
    import M1(x)
    y = x

In this example, there is no way to supply an explicit type signature for y since T is not in scope. Whether or not T is explicitly exported, module M2 knows enough about T to correctly type check the program.

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.

5.6  Standard Prelude

Many of the features of Haskell are defined in Haskell itself as a library of standard datatypes, classes, and functions, called the "Standard Prelude." In Haskell , the Prelude is contained in the module Prelude. There are also many predefined library modules, which provide less frequently used functions and types. For example, complex numberss, arrays, and most of the input/output are all part of the standard libraries. These are defined in Part II Separating libraries from the Prelude has the advantage of reducing the size and complexity of the Prelude, allowing it to be more easily assimilated, and increasing the space of useful names available to the programmer.

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 consulting the source code of the Prelude.

5.6.1  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 entities defined in the Prelude to be selectively imported, just like those from any other module.

The semantics of the entities in Prelude is specified by a reference implementation of Prelude written in Haskell , given in Chapter 8. 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 Chapter 8. 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.

Chapter 8 defines the module Prelude using several other modules: PreludeList, PreludeIO, and so on. These modules are not part of Haskell 98, and they cannot be imported separately. They are simply there to help explain the structure of the Prelude module; they should be considered part of its implementation, not part of the language definition.

5.6.2  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( null, nonNull ) where
    import Prelude hiding( null ) 
    null, nonNull :: Int -> Bool
    null    x = x == 0
    nonNull x = not (null x)

Module A redefines null, and contains an unqualified reference to null on the right hand side of nonNull. The latter would be ambiguous without the hiding(null) on the import Prelude statement. Every module that imports A unqualified, and then makes an unqualified reference to null must also resolve the ambiguous use of null 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 Prelude()
    import MyPrelude
    f x = (x,x)
    g x = (,) x x
    h x = [x] ++ []

the explicit import Prelude() declaration prevents the automatic import of Prelude, while the declaration import MyPrelude brings the non-standard prelude into scope. The special syntax for tuples (such as (x,x) and (,)) and lists (such as [x] and []) continues to refer to the tuples and lists defined by the standard Prelude; there is no way to redefine the meaning of [x], for example, in terms of a different implementation of lists. On the other hand, the use of ++ is not special syntax, so it refers to ++ imported from MyPrelude.

It is not possible, however, to hide instance declarations in the Prelude. For example, one cannot define a new instance for Show Char.

5.7  Separate Compilation

Depending on the Haskell implementation used, separate compilation of mutually recursive modules may require that imported modules contain additional information so that they may be referenced before they are compiled. Explicit type signatures for all exported values may be necessary to deal with mutual recursion. The precise details of separate compilation are not defined by this report.

5.8  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. Instead, they must use push, pop, and empty to construct such values.

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 (_:s)) = Stk s
    empty = Stk []

The Haskell 98 Report
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December 2002