Register

# 6.2. Lazy Lists¶

## 6.2.1. Infinite Sequences¶

Implementing call-by-name

Macro-expansion may be implemented with a double textual substitution (as in the C++ pre-processor) or with a single substitution and dynamic scoping. The result is the evaluation of the entire function body in the caller's environment. But how to implement call-by-name? How to evaluate the arguments in the caller’s environment but the rest of the body in the callee’s environment?

Instead of simply passing a textual representation of the argument, we pass in a parameterless anonymous function that returns the argument. Such an anonymous function is called a thunk.

Understanding the difference between an argument that is evaluated and a thunk is to understand the difference between 7 and function () { return 7; }.

Instead of evaluating the argument before calling the function and using that value in the function, every time a parameter is referenced in the function body, the thunk is thawed to obtain the argument’s value.

Call-by-name lists

To illustrate the use of thunks, we will implement call-by-name lists, which are similar to the way they are used by default in Haskell and, as a programmer-chosen option, in Python. Call-by-name lists essentially give you lazy lists, which can also be thought of as "infinite sequences".

Below is a partial code listing for a JavaScript module for infinite sequences called the is module.

The constructor for sequences (i.e., the cons function) takes two arguments, namely the element we want at the head of the sequence and a thunk that will return the tail of the sequence if we ever need to go beyond the first element. For simplicity, we will only manipulate sequences of integers.

Here is the code for some of the functions in the is module.

// return the sequence frozen in the given thunk
// this function is NOT exported
var thaw = function (thunk) {
return thunk();
};

// construct a new sequence made of the given integer and thunk
var cons = function (n,thunk) {
return [n, thunk];
};

// get the first integer in the  sequence
var hd = function (seq) {
return seq[0];
};

// return the sequence that is obtained by deleting the head of the given sequence
var tl = function (seq) {
return thaw(seq[1]);
};

// return the list containing the first n integers in the given sequence
var take = function (seq,n) {
if (n === 0)
return [];
else {
var result = take(tl(seq), n - 1).slice(0);
result.unshift(hd(seq));
return result;
}
};


In addition to the functions listed above, the is module has some helper functions that are "infinite analogues" to their counterparts in finite lists (our fp module). Think about how the set of question marks should be filled in to complete these functions before proceeding to the practice problems

// return the sequence of successive integers starting at n
var from = function (n) {
return cons(n, function () { ?????? });
};

// return the sequence obtained by removing the first n integers from the given sequence
var drop = function (seq,n) {
if (n === 0)
return seq;
else {
return drop( ?????? );
}
};

// return a new sequence obtained by mapping the given function onto the given sequence
var map = function (f,seq) {
return cons (  ?????? );

};

// return a new sequence obtained by filtering the given sequence with the given predicate
var filter = function (pred,seq) {
if (pred(hd(seq))) {
return cons ( ?????? );
} else {
return ??????;
}
};

// return a new sequence obtained by repeatedly applying the given function to the
// previous term of the sequence (starting with the given integer).   That is, return
// the sequence n, f(n), f(f(n)), f(f(f(n))), ...
var iterates = function (f,n) {

return cons(n, ?????? );
};


Call-by-need

What's the difference between our call-by-name implementation of infinite sequences and the way it is done in Haskell? In Haskell, the analogue of the is.tl and is.take functions are done with call-by-need instead of call-by-name. In call-by-need, the value returned by a thunk is stored (that is, cached) after it is thawed for the first time. This is much more efficient since it never results in a thunk being thawed more than once..

This problem will help you better understand code that creates call-by-name infinite sequences.

## 6.2.2. Practice With Infinite Sequences¶

This problem will help you write recursive code to process infinite sequences. To earn credit for it, you must complete this randomized problem correctly three times in a row.

## 6.2.3. Practice With Infinite Sequences (2)¶

This problem reviews recursive definitions of sequences. To earn credit for it, you must complete this randomized problem correctly three times in a row.

## 6.2.4. Practice With Infinite Sequences (3)¶

This problem deals with one more example of a recursive definition of a sequence.