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.
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))
})
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)
pip install adt.py
Copyright (c) 2017 Eric Gjertsen [email protected]
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