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). 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 Section 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 the Haskell Library Report).
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 ; 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, 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.
| 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:
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).
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).
| 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.
Exactly which entities are to be imported can be specified in one of the following three ways:
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.
It is an error to hide an entity that is not, in fact, exported by the imported module.
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)
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.
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 |
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>>"
A qualified name is written as modid.name (Section 2.4). A qualified name is brought into scope:
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.
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.
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.
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 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.
Appendix A 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.
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.
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 []