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Module export

export syntax​

Code example in this chapter:

module Shapes
where

import Data.Number (pi)
import Data.Ord (abs)

data Shape = Circle Number Number Number
           | Rectangle Number Number Number Number

surface :: Shape -> Number
surface (Circle _ _ r) = pi * r * r
surface (Rectangle x1 y1 x2 y2) = (abs $ x2 - x1) * (abs $ y2 - y1)

export all (implicitly)​

The following will export all:

module Shapes where
  • types: Shape
  • value constructors: Circle and Rectangle
  • functions: surface
Haskell reference

If the export list is omitted, all values, types and classes defined in the module are exported, but not those that are imported. https://www.haskell.org/onlinereport/modules.html

export all value constructors​

The following code with export all value constructors of type Shape: Circle and Rectangle:

module Shapes
(Shape (..)) where

not export value constructors​

The following code will only export the data type Shape, as a result the module which imports it, will not be able to construct value of Shape.

module Shapes
(Shape) where

Not exporting the value constructors of a data types makes them more abstract in such a way that we hide their implementation. Also, whoever uses our module can't pattern match against the value constructors.

export function with its required data type​

PureScript does not allow to export function without its required data type. Like the code below, if we don't export Shape, we will get compile error:

module Shapes
(surface, Shape) where

export partial constructors?​

In PureScript (like Haskell), data constructors are all or nothing when it comes to exports. You can:

  • Export just the type (hiding all constructors)
  • Export the type and all its constructors

But you cannot selectively export only some constructors. The code below will not compile:

module Shapes
(Shape(Circle)) where
warning

You cannot export only specific constructors of a data type in PureScript.

reference doc​