Import
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:
- 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 Mod exports four functions named x, y, z, and (+++)...
| Import command | What is brought into scope | Notes |
|---|---|---|
import Mod
|
x, y, z, (+++), Mod.x, Mod.y, Mod.z, (Mod.+++)
|
(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, (Mod.+++)
|
(Only x, y, and (+++), no z.)
|
import qualified Mod
|
Mod.x, Mod.y, Mod.z, (Mod.+++)
|
(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, (Mod.+++)
|
(x and y are hidden.)
|
import qualified Mod hiding (x,y)
|
Mod.z, (Mod.+++)
|
(x and y are hidden.)
|
import Mod as Foo
|
x, y, z, (+++), Foo.x, Foo.y, Foo.z, (Foo.+++)
|
(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, (Foo.+++)
|
(Only qualified names, using new qualifier.) |
import qualified Mod as Foo (x,y)
|
Foo.x, Foo.y
|
(Only qualified versions of x and y, 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)
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.
See also
- Import modules properly - Some thoughts that may help to decide what form of the import statement to use.