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predicates
selfsame edited this page Jan 18, 2015
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A predicate function takes one argument and returns a boolean.
Dispatch rules require predicate functions for each argument.
def number(x): return isinstance(x, (int, float))
def zero(x): return x == 0
@check(zero, number)
def divide(a, b):
report("The fabric of the universe grinds to a halt!")
return False
@given(number, number)
def divide(a, b):
return a / b
For general purpose use, predicates can serve as a type signature. mud.core
includes the standard type predicates:
anything number integer string function
sequential dictionary module undefined empty
Predicates are easy to compose into new predicates. The following composition functions are provided:
a(even, number)
a(non(zero), even, number)
To avoid import dependencies, predicates can be bound to strings.
bind.predicate("zero", zero)
#predicate guards can take bound strings
@given("zero")
def square(n): return 0
#composition functions can take bound strings
a(non("zero"), "number")
string representations are looked up on invokation. A game/
module that rebinds a predicate will replace it for every bound usage in the application.
predicate composition works well with the entity component compositions. (the provided has
function is a generator for predicate key existence functions)
bind.predicate("thing", dictionary)
bind.predicate("positioned", a(has("x"), has("y"))
bind.predicate("dimensional", a(has("width"), has("height"))
bind.predicate("rectangle", a("dimensional", "positioned", "thing"))
@given("rectangle")
def left(e):
return e["x"] - e["width"]/2