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adt.py

Simple algebraic data types in Python

  • records (product types)
  • tagged unions (sum types)
  • simple tag-based pattern matching
  • immutable structures
  • records and tagged unions can be pickled/unpickled
  • python 2 and 3 compatible

Tip of the hat to union-type, a javascript library with similar aims and syntax.

Tagged unions

Point = Type("Point", [int, int])
Rectangle = Type("Rectangle", [Point, Point])
Circle = Type("Circle", [int, Point])

# a union is defined as a sequence of types
Shape = [ Rectangle, Circle ]

# that then can be matched on
area = match(Shape, {
  Rectangle: (lambda (t,l), (b,r): (b - t) * (r - l)),
  Circle: (lambda r, _: math.pi * (r**2))
})

rect = Rectangle( Point(0,0), Point(100,100) )
area(rect)  # => 10000

circ = Circle( 5, Point(0,0) )
area(circ)  # => 78.539816...

Note that this is python 2 syntax. Python 3 sadly removed destructuring parameters in lambdas, so you have to do something like:

area = match(Shape, {
  Rectangle: (lambda tl, br: (br[0] - tl[0]) * (br[1] - tl[1])),
  Circle: (lambda r, _: math.pi * (r**2))
})

Records

 Point = Record("Point", {'x': int, 'y': int})
 
 # types can be composed of records
 Rectangle = Type("Rectangle", [Point, Point])
 
 # and records composed of types
 Circle = Record("Circle", {'radius': int, 'center': Point} )

 Shape = [ Rectangle, Circle ]

 # including records composed of union types
 ScaledShape = Record("ScaledShape", {'shape': Shape, 'scale': int})
 
 p1 = Point(x=1,y=2)
 p2 = Point(x=4,y=6)
 rect = Rectangle( p1, p2 )
 srect = ScaledShape(shape=rect, scale=10)

To install

pip install adt.py

License

Copyright (c) 2017 Eric Gjertsen [email protected]

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.