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(rm cases that realistically do not get used)
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| <hask>Mod.x</hask>
 
| <hask>Mod.x</hask>
 
| (Only <hask>x</hask>, only qualified.)
 
| (Only <hask>x</hask>, only qualified.)
|-
 
| <hask>import Mod hiding ()</hask>
 
| <hask>x, y, z, Mod.x, Mod.y, Mod.z</hask>
 
| (Same as with no <hask>hiding</hask>.)
 
 
|-
 
|-
 
| <hask>import Mod hiding (x)</hask>
 
| <hask>import Mod hiding (x)</hask>
 
| <hask>y, z, Mod.y, Mod.z</hask>
 
| <hask>y, z, Mod.y, Mod.z</hask>
 
| (<hask>x</hask> is hidden.)
 
| (<hask>x</hask> is hidden.)
|-
 
| <hask>import qualified Mod hiding ()</hask>
 
| <hask>Mod.x, Mod.y, Mod.z</hask>
 
| (Same as with no <hask>hiding</hask>.)
 
 
|-
 
|-
 
| <hask>import qualified Mod hiding (x)</hask>
 
| <hask>import qualified Mod hiding (x)</hask>

Revision as of 17:26, 13 February 2007

The
import
statement is used to import functions and other definitions from another module. In the simplest case, you just write
import Data.Maybe
to import the named module (in this case
Data.Maybe
).

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:

Suppose that module
Mod
exports threefunctions named
x
,
y
and
z
. In that case:
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)
x, Mod.x
(Only
x
, no
y
or
z
.)
import qualified Mod
Mod.x, Mod.y, Mod.z
(Only qualified versions; no unqualified versions.)
import qualified Mod (x)
Mod.x
(Only
x
, only qualified.)
import Mod hiding (x)
y, z, Mod.y, Mod.z
(
x
is hidden.)
import qualified Mod hiding (x)
Mod.y, Mod.z
(
x
is 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)
x, Foo.x
(Only import
x
.)
import qualified Mod as Foo
Foo.x, Foo.y, Foo.z
(Only qualified names, using new qualifier.)