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Import

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(Added notes about the Prelude.)
Current revision (14:16, 28 January 2012) (edit) (undo)
(explicitely say multiple imports are allowed; do some minor reorganizing)
 
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[[Category:Language]]
[[Category:Language]]
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The <hask>import</hask> statement is used to import functions and other definitions from another module. In the simplest case, you just write
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[[Category:Glossary]]
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The <hask>import</hask> statement is used to import functions and other definitions from another module. The shortest form of the import statement is
<haskell>
<haskell>
import Data.Maybe
import Data.Maybe
</haskell>
</haskell>
-
to import the named module (in this case <hask>Data.Maybe</hask>).
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that imports the named module (in this case <hask>Data.Maybe</hask>).
-
However, in more complicated cases, the module can be imported qualified, with or without hiding, and with or without renaming. Getting all of this straight in your head is quite tricky, so here is a table (lifted directly from the language reference manual) that roughly summarises the various possibilities:
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However, there are more options:
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Suppose that module <hask>Mod</hask> exports three functions named <hask>x</hask>, <hask>y</hask> and <hask>z</hask>. In that case:
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# Modules can be imported '''qualified''' (forcing an obligatory namespace qualifier to imported identifiers).
 +
# Some identifiers can be skipped via the '''hiding''' clause.
 +
# The module namespace can be renamed, with an '''as''' clause.
 +
 
 +
Getting all of this straight in your head is quite tricky, so here is a table (lifted directly from the language reference manual) that roughly summarises the various possibilities:
 +
 
 +
Supposing that the module <hask>Mod</hask> exports three functions named <hask>x</hask>, <hask>y</hask> and <hask>z</hask>...
{|
{|
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|}
|}
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Note also that, by default, ''every module'' implicitly imports <hask>Prelude</hask>. However, if you add an ''explicit'' import declaration for the prelude, this turns off the implicit one. Thus, if you wanted (for example) to write a module that redefines <hask>zip</hask> you could do
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Note that multiple import statements for the same module are also allowed, so it is possible to mix and match styles if its so desired (for example, importing operators directly and functions qualified)
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===Hiding Prelude===
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By default, ''every module'' implicitly imports <hask>Prelude</hask>. However, if you add an ''explicit'' import declaration for the prelude, this turns off the implicit one. Thus, if you wanted (for example) to write a module that redefines <hask>zip</hask> you could do
<haskell>
<haskell>
module Mod where
module Mod where
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Note that any module ''using'' a module that redefines prelude functions will need to import either the prelude or the other module (or maybe both) qualified and/or with hiding for the same reason.
Note that any module ''using'' a module that redefines prelude functions will need to import either the prelude or the other module (or maybe both) qualified and/or with hiding for the same reason.
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== See also ==
 +
 +
* [[Import modules properly]] - Some thoughts that may help to decide what form of the import statement to use.

Current revision

The
import
statement is used to import functions and other definitions from another module. The shortest form of the import statement is 
import Data.Maybe
that imports the named module (in this case
Data.Maybe
).

However, there are more options:

  1. Modules can be imported qualified (forcing an obligatory namespace qualifier to imported identifiers).
  2. Some identifiers can be skipped via the hiding clause.
  3. The module namespace can be renamed, with an as clause.

Getting all of this straight in your head is quite tricky, so here is a table (lifted directly from the language reference manual) that roughly summarises the various possibilities:

Supposing that the module
Mod
exports three functions named
x
,
y
and
z
...
Import command What is brought into scope Notes
import Mod
x, y, z, Mod.x, Mod.y, Mod.z
 (By default, qualified and unqualified names.)
import Mod ()
 (Nothing!) (Useful for only importing instances of typeclasses and nothing else)
import Mod (x,y)
x, y, Mod.x, Mod.y
 (Only
x
and
y
, no
z
.)
import qualified Mod
Mod.x, Mod.y, Mod.z
 (Only qualified versions; no unqualified versions.)
import qualified Mod (x,y)
Mod.x, Mod.y
 (Only
x
and
y
, only qualified.)
import Mod hiding (x,y)
z, Mod.z
 (
x
and
y
are hidden.)
import qualified Mod hiding (x,y)
Mod.z
 (
x
and
y
are hidden.)
import Mod as Foo
x, y, z, Foo.x, Foo.y, Foo.z
 (Unqualified names as before. Qualified names use
Foo
instead of
Mod
.)
import Mod as Foo (x,y)
x, y, Foo.x, Foo.y
 (Only import
x
and
y
.)
import qualified Mod as Foo
Foo.x, Foo.y, Foo.z
 (Only qualified names, using new qualifier.)

Note that multiple import statements for the same module are also allowed, so it is possible to mix and match styles if its so desired (for example, importing operators directly and functions qualified)

1 Hiding Prelude

By default, every module implicitly imports
Prelude
. However, if you add an explicit import declaration for the prelude, this turns off the implicit one. Thus, if you wanted (for example) to write a module that redefines
zip
you could do 
module Mod where
 
import Prelude hiding (zip)
 
zip = {- ... -}
Without the
import
statement, you could receive a compile-time error about an 'ambiguous use of
zip
'. A slightly more messy alternative is to do 
module Mod where
 
import qualified Prelude as P
 
zip = {- ... -}
This has the disadvantage that (say) '
P.show (2 P.+ 3 P.* 3) P.++ "abc"
' is very messy to read. Typically a module only redefines a few prelude functions, and it's simpler to just hide the ones you don't want to clash with.

Note that any module using a module that redefines prelude functions will need to import either the prelude or the other module (or maybe both) qualified and/or with hiding for the same reason.

2 See also

Retrieved from "http://www.haskell.org/haskellwiki/Import"

Categories: Language | Glossary