Thunks and Streams Assignment
Contents
Motivation
It is all too easy to make a simple mistake on the Streams Lab and have it burn way to much of ones time. We will build some utility functions that will hopefully make our code more clear, as well as raise better errors sooner when we make a mistake.
Code To Use
Code To Implement
file: | src/main/racket/hw4/hw4.rkt | |
functions: | thunk? thunk-that dethunk-that ... |
Thunk Utilities
A thunk is a 0 argument function. We will write functions to check if an expression is a thunk, a MACRO to create a thunk from an expression, and a function to evaluate a thunk.
thunk?
define a function thunk?
which returns whether the specified parameter is a thunk or not.
(define (thunk? th) (error 'not-yet-implemented))
true: #t
false: #f
is a function: procedure?
number of parameters: procedure-arity
thunk-that
define a MACRO thunk-that
which creates thunk
(define-syntax-rule (thunk-that e) (error 'not-yet-implemented))
dethunk-that
define a function dethunk
which takes a thunk parameter e
and returns the result of invoking e
.
(define (dethunk-that e) (error 'not-yet-implemented))
If thunking and expression wraps an expression in a single argument function, then de-thunking is simply calling that function.
NOTE: It may seem unnecessary to use dethunk-that
when implementing Lab4, when you could simply (thunk)... that is "call the thunk". Still, you are encouraged to use dethunk-that
as a bit of verbosity can sometimes help in debugging a sea already full of parentheses.
Stream Utilities
value-next-stream-pair-from-stream
define a function stream-next
which takes a stream parameter and returns a pair of values for the next value and the next stream.
next-value-from-stream
provided function next-value-from-stream
which takes a stream parameter and returns the next value of that stream.
(define (value-from-stream s) (let-values ([(value s-prime) (value-next-stream-pair-from-stream s)]) value))
next-stream-from-stream
provided function next-stream-from-stream
which takes a stream parameter and returns the next stream of that stream.
(define (next-stream-from-stream s)
(let-values ([(value s-prime) (value-next-stream-pair-from-stream s)]) s-prime))
stream-cons-ensuring-stream-prime-is-thunk
define a function stream-cons-ensuring-stream-prime-is-thunk
which takes two parameters value
and s-prime
. if the specified s-prime
parameter is not a thunk then an error should be raised:
(raise (error "not a thunk" stream-prime))
If s-prime
is a thunk it should simply cons value
and stream-prime
. Do NOT create a thunk.
(define (stream-cons-ensuring-stream-prime-is-thunk value s-prime) (error 'not-yet-implemented))
Stream App
flip-flop-stream
define flip-flop-stream
which produces #t #f #t #f #t #f #t #f...