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Zef vs Python Lists & Tuples

We use these terms with a somewhat different meaning than their connotation in Python.

Python​

These Python types say a lot about the representation of the data.

  • List: a mutable data structure. It's type says something about the data representation: contiguously stored pointers to Python object, it cannot be lazy
  • Tuple: an immutable data structure, very similar to a Python list otherwise

Zef Lists and Tuples: Logical Types​

zef.List and zef.Tuple say something about the logical form of the data. A given value could be both a List and a Tuple of a certain kind.

e.g. 

T1 = List[Int]            # the set of all 1d sequences where each element is an Int  
[1, 5] | is_a[T1]         # => True  
[1, 5, 42] | is_a[T1]     # => True  
  
T2 = Tuple[Int, Int, Int]  
[1, 5] | is_a[T2]         # => False: does not have three elements  
[1, 5, 42] | is_a[T2]     # => True

Laziness​

Specifying that a value is a Zef List or Tuple says nothing about whether it is lazy or "fully evaluated", i.e. the computational policy. This means that also a lazy sequence 

# infinite (lazy) seuquence of Fibonacci numbers  
fib = (0, 1) | iterate[lambda p: (p[1], sum(p))] | map[second]

is considered a List[Int].
Side note for the experts: asking this question for infinite sequences may be undecidable.

Streams are also considered to be Lists over time. e.g. a Stream[Int] is a subtype of List[Int].

List Subtyping​

A single type can be specified as a subtype for a list: 

List[T1]

can be understood as a the set of all lists where each element is of type T1. It says nothing about the length of the list.
The bare form List is equivalent to List[Any], i.e. is the union of all subtyped lists.

Tuple Subtyping​

A list of subtypes can be specified for a Tuple 

T3 = Tuple[String, Float]      
['hi', 3.2] | is_a[T3]        #  ✅  
['hi', 3.2, 1] | is_a[T3]     #  ❌ length does not match

The element at each position needs to match the specified type.

Relation to Python's Typing Module​

import typing

Further Material​