The code from page 113 of Lisp in small pieces seems can't run on r5rs environment of racket:
(let ((name "Hemo"))
(set! winner (lambda () name))
(set! set-winner! (lambda (new-name) (set! name new-name) name ))
(set-winner! "Me")
(winner) )
and:
(let ((name "Nemo"))
(set! winner (lambda () name))
(winner) )
both got error:
cannot set variable before its definition
variable: winner
is the code of this book wrong?or I should not use scheme?I have to use the lisp this book defined to run this code?Thanks!
You need to bind the variable using define or lambda. For example:
(let ((name "Hemo"))
(define winner #t)
(define set-winner! #t)
(set! winner (lambda () name))
(set! set-winner! (lambda (new-name) (set! name new-name) name ))
(set-winner! "Me")
(winner) )
set! is converted into an scode whose semantics is to mutate a location of memory, not to bind new locations. Forms like let are syntactic sugar for the combination of both operations (bind+mutation) and, because they are so common, Queinnec inserted an scode, fixlet, for this combination.
All these things are explained in the book of Queinnec. Go on reading!
Related
TL;DR: What does call/cc do, (semi-)formally speaking?
The long version: I'm vaguely familiar with continuations and call/cc, but I don't have a strong formal understanding. I would like one.
In the SICP video lectures we are presented with the substitution model and a metacircular interpreter. In Shriram Krishnamurti's programming language course we are shown an environment-and-store-passing style. In the compiler course I took in uni I evaluated Java expressions by manipulating a stack.
What's the simplest evaluation model in which call/cc can be expressed, and how do you express call/cc in it?
TL;DR call/cc lets you tap into Schemes internal code so that you can use continuations without writing your code in continuation passing style. The best evaluation model is the substitution model given that you don't use set and view it from CPS code
Imagine this little program.
(define (sum-list lst)
(cond ((null? lst) 0)
((number? (car lst)) (+ (car lst) (sum-list (cdr lsr))))
(else (sum-list (cdr lsr)))))
(display (sum-list '(1 2 3 4))) ; displays 10
Imagine you want the result to be 1337 if you hit the else term. How would you go about that without rewriting the whole thing? You can't. However most Scheme implementation does CPS conversion to your code where changing it is trivial:
(define (sum-list& lst k)
(null?& lst
(lambda (nlst?)
(if& nlst?
(lambda ()
(k 0))
(lambda ()
(car& lst
(lambda (car-lst)
(number?& car-lst
(lambda (ncl?)
(if& ncl?
(lambda ()
(cdr& lst
(lambda (clst)
(sum-list& clst
(lambda (sclst)
(+& car-lst sclst k))))))
(lambda ()
(cdr& lst
(lambda (clst)
(sum-list& clst k))))))))))))))
(sum-list& '(1 2 3 4)
(lambda (sum)
(display& sum halt)))
cond is a macro so it is the if& calls you see. I know what you're thinking. Why not tell the programmers to do it like this in the first place? Just kidding!
If you change the second continuation in the second if& to (lambda () (k 1337)) you're done. As beautiful as CPS is it is horrible to read and write, but since the compiler does this anyway couldn't there be a way to be able to write your code in the first way and still get to code control flow? The best of two worlds is enabled with call/cc. call/cc is this in CPS:
(define (call/cc& f& continuation)
(define (exit& value actual-continuation)
(continuation value))
(f& exit& continuation))
It doesn't do much at all. It passes exit& which ignores the real continuation of the program when called and calls the value with the original continuation of the call/cc& call. With the list '(1 2 3 #f) you'd have 3 nested continuations waiting to add with the result but all of that needs to be cancelled. If you get to choose the value of the continuation before that calculation it automatically is cancelled.
And that is it. When you write your code with it you don't need to do full CPS, only the continuation you want thought call/cc like this:
(define (sum-list lst)
(call/cc
(lambda (k)
(define (helper lst)
(cond ((null? lst) 0)
((number? (car lst))
(+ (car lst) (helper (cdr lst))))
(else (k 1337))))
(helper lst))))
This gets transformed to this in CPS:
(define (sum-list& lst k)
(call/cc&
(lambda (k& real-k)
(define (helper& lst k)
(null?& lst
(lambda (nlst?)
(if& nlst?
(lambda ()
(k 0))
(lambda ()
(car& lst
(lambda (car-lst)
(number?& car-lst
(lambda (ncl?)
(if& ncl?
(lambda ()
(cdr& lst
(lambda (clst)
(helper& clst
(lambda (sclst)
(+& car-lst sclst k))))))
(lambda ()
(k& 1337 real-k))))))))))))
(helper& lst real-k))
k))
(sum-list& '(1 2 3 4)
(lambda (sum)
(display& sum halt)))
call/cc can always be avoided. Our example could have been rewritten to return 1337 like this:
(define (sum-list lst)
(define (helper lst sum)
(cond ((null? lst) sum)
((number? (car lst)) (helper (cdr lst) (+ sum (car lst))))
(else 1337)))
(helper lst 0))
This is tail recursive and instead of creating continuations it accumulates. For completeness here is the CPS version of it:
(define (helper& lst sum k)
(null?& lst
(lambda (nlst)
(if& nlst
(lambda () (k sum))
(lambda ()
(car& lst
(lambda (cl)
(number?& cl
(lambda (ncl?)
(if& ncl?
(lambda ()
(cdr& lst
(lambda (cdrl)
(+& sum
cl
(lambda (scl)
(helper& cdrl
scl
k))))))
(lambda () (k 1337))))))))))))
(define (sum-list& lst k)
(helper& lst 0 k))
I found this excellent presentation (in German), which answered my question: https://www.youtube.com/watch?v=iuaM9-PX1ls
To evaluate the lambda calculus with call/CC, you pass around an evaluation context consisting of an environment (as usual) and a call stack. A call to call/cc creates a special function-like continuation object which stores the evaluation context. The result of applying the special object to an expression expr is the result of interpreting expr in the evaluation context captured in the continuation object.
TL;DR: you can implement call/cc with an environment-and-call-stack-passing interpreter.
If you want to also thread around a mutable store the continuation objects should not capture it. Rather, when invoking a continuation you pass the store as an argument to the interpretation in the restored evaluation context. (The store can be of a linear type.)
Here is one such implementation in Haskell. Here's a sample expression you may want to evaluate: interpret 0 (Application (Lambda (1, (CallCC (Lambda (2, (Application (Variable 2) (Lambda (3, (Variable 4))))))))) (Lambda (4, (Variable 5)))).
(The numbers are just plain old names, not (e.g.) De Bruijn indices. If you wish to use characters or strings, change type Name = Integer to type Name = Char.)
Note especially interp applied to CallCC and InvokeContinuation as well as continue applied to ContinuationArgument.
import qualified Data.Map as Map
type Name = Integer
type NameAndBody = (Name, LambdaCallCC)
data LambdaCallCC
= Lambda NameAndBody
| Variable Name
| InvokeContinuation EvaluationContext LambdaCallCC
| CallCC LambdaCallCC
| Application LambdaCallCC LambdaCallCC
deriving Show
type Environment = Map.Map Name NameAndBody
type EvaluationContext = (CallStack, Environment)
type CallStack = [Frame]
data Frame
= FunctionPosition LambdaCallCC
| ArgumentPosition NameAndBody
| ContinuationArgument EvaluationContext
deriving Show
type Fail = (Name, EvaluationContext)
interpret :: Name -> LambdaCallCC -> Either Fail NameAndBody
interpret thunkVarName expression = interp [] Map.empty expression
where interp stk env (Lambda nameAndBody)
= continue stk env nameAndBody
interp stk env (Variable name)
= case Map.lookup name env of
Nothing -> Left (name, (stk, env))
Just e -> continue stk env e
interp stk env (Application e1 e2)
= interp (FunctionPosition e2 : stk) env e1
interp stk env (CallCC expr)
= interp stk env (Application expr
(Lambda (thunkVarName,
(InvokeContinuation
(stk, env)
(Variable thunkVarName)))))
interp stk env (InvokeContinuation capturedContext expr)
= interp [ContinuationArgument capturedContext] env expr
continue [] env value
= Right value
continue ((FunctionPosition expr) : frames) env value
= interp ((ArgumentPosition value) : frames) env expr
continue ((ArgumentPosition (name, body)) : frames) env value
= interp frames (Map.insert name value env) body
continue ((ContinuationArgument (stk, env)) : nil) _ value
= interp stk env (Lambda value)
I'm trying to make a basic journal entry language in racket for a class on DSLs (specifically beautiful racket, the training version). I have a correct parser and reader, but I am having difficulty with my macros. Below is my expander code. I am trying to have my DSL be functional rather than imperative (I know I could have a global ledger value that I update, but it is not in the spirit of this project).
I have included the code for the important functions in this module. A lot of them are based off of this tutorial: https://beautifulracket.com/bf/a-functional-expander.html
Right now, when I run my code on this text:
[1920-02-19<equipment-20000><cash-10000,stock-10000>]
[2020-12-20<insurance-500><cash-500>]
I get this output from my display calls:
(#<date 1920-02-19> debits credits)()(#<date 2020-12-20> debits credits)#<void>#<void>
It looks like my lambda is not returning the updated ledger in my journal-entry macro. My knowledge isn't great when it comes to Racket, so I'm not sure what I'm doing wrong here. Any tips?
(define (fold-funcs apl ac-funcs)
(for/fold ([current-apl apl])
([ac-func (in-list ac-funcs)])
(ac-func current-apl)
(display current-apl)))
(define-macro (ac-line ENTRIES ...)
#'(begin
(define ledger empty)
(set! ledger (fold-funcs ledger (list ENTRIES ...)))
(display ledger)))
(provide ac-line)
(define-macro (journal-entry "[" INFO ... "]")
#'(lambda (ledger)
(define entry (list INFO ...))
(define dt (first entry))
(set! entry (rest entry))
(define d (first entry))
(set! entry (rest entry))
(define c (first entry))
(define e (list dt d c))
(display e)
(set! ledger (cons e ledger))
ledger
))
(provide journal-entry)
In an attempt to emulate simple OOP in scheme (just for fun), I have caught myself repeating the following pattern over and over:
(define my-class ; constructor
(let ((let-for-name-encapsulation 'anything))
; object created from data is message passing interface
(define (this data)
(lambda (m)
(cond ((eq? m 'method1) (method1 data))
((eq? m 'method2) (method2 data))
(else (error "my-class: unknown operation error" m)))))
;
(define (method1 data)
(lambda (arg1 ...)
... )) ; code using internal 'data' of object
;
(define (method2 data)
(lambda (arg2 ...)
... ))
;
; returning three arguments constructor (say)
;
(lambda (x y z) (this (list 'data x y z)))))
I decided to wrap everything inside a let ((let-for-name-encapsulation ... so as to avoid leaking names within the global environment while still
being able to use the define construct for each internal function name, which enhances readability. I prefer this solution to the unsightly construct (let ((method1 (lambda (... but I am still not very happy because of the somewhat artificial let-for-name-encapsulation. Can anyone suggests something simple which would make the code look even nicer?. Do I need to learn macros to go beyond this?
I use that pattern often, but you don't actually need to define any variables:
(define binding
(let ()
(define local-binding1 expression)
...
procedure-expression)))
I've seen it in reference implementations of SRFIs so it's a common pattern. Basically it's a way to make letrec without the extra identation and lambdas. It can easily be made a macro to make it even flatter:
(define-syntax define/lexical
(syntax-rules ()
((_ binding body ...)
(define binding
(let ()
body ...)))))
;; test
(define/lexical my-class
(define (this data)
(lambda (m)
(cond ((eq? m 'method1) (method1 data))
((eq? m 'method2) (method2 data))
(else (error "my-class: unknown operation error" m)))))
(define (method1 data)
(lambda (arg1 ...)
... )) ; code using internal 'data' of object
(define (method2 data)
(lambda (arg2 ...)
... ))
;; returning three arguments constructor (say)
(lambda (x y z) (this (list 'data x y z))))
;; it works for procedures that return procedures as well
(define/lexical (count start end step)
(define ...)
(lambda ...))
Of course you could use macros to simplify your object system as well.
I want to be able to take a procedure and see what it looks like. Is this possible?
For example, let's say I have:
(define (some-func x)
(+ x 1))
What I want to do is apply some amazing function (say, stringify) to some-func and be able to look at its guts.
\> (stringify some-func)
"(lambda (x) (+ x 1))"
I haven't found any Racket libraries that do it. Can it be done?!
In R6RS, there is no sure way to determine if two procedures are equivalent; even an expression like (let ((p (lambda () 42))) (eqv? p p)) is not guaranteed to be true.
R7RS addresses that by using the concept of "location tags", where each lambda expression generates a unique location tag. Then eqv? works for procedures by comparing location tags: thus, (let ((p (lambda () 42))) (eqv? p p)) is true, and (eqv? (lambda () 42) (lambda () 42)) is false.
There is no reliable way to get the source of a procedure (many implementations macro-expand and compile the procedures, discarding the original source), and even if you could, you could not use it to compare if two procedures are "equal", because of closures (and that two procedures could have the same "source" but have their free variables bound to different things). For example, consider the two expressions (let ((x 1)) (lambda () x)) and (let ((x 2)) (lambda () x)). They have the same "source", but nobody in their right mind would claim that they are equivalent in any way.
Note, you could easily implement a define alternative to keep the source around. You don't avoid the lexical issues but, modulo that, you've got something with limited use.
(define name->source-mapping '())
(define (name->source name)
(cond ((assq name name->source-mapping) => cdr)
(else #f)))
(define (name->source-extend name source)
(set! name->source-mapping (cons (cons name source) name->source-mapping))
(define-syntax define-with-source
((_ (name args ...) body1 body2 ...)
(define name
(begin (name->source-mapping-extend 'name '(lambda (args ...) body1 body2 ...))
name->source-mapping))
(lambda (args ...) body1 body2 ...)))))
[Above does not replace (define name value) syntax; consider the above an example only.]
This seems to work, it's a macro that expands to successive integers depending on how many times it has been expanded.
;; Library (test macro-state)
(library
(test macro-state)
(export get-count incr-count)
(import (rnrs))
(define *count* 0)
(define (get-count) *count*)
(define (incr-count) (set! *count* (+ *count* 1)))
)
;; Program
(import (rnrs) (for (test macro-state) expand))
(define-syntax m
(lambda (x)
(syntax-case x ()
((m) (begin (incr-count) (datum->syntax #'m (get-count)))))))
(write (list (m) (m) (m)))
(newline)
;; prints (1 2 3)
But it's clumsy to me because the macro state *count* and the macro m itself are in different modules. Is there a better way to do this in r6rs, preferably one that doesn't split the implementation over two modules?
EDIT
I should make it clear that although this example is just a single macro, in reality I'm looking for a method that works when multiple macros need to share state.
You can make the state local to the macro transformer:
(define-syntax m
(let ()
(define *count* 0)
(define (get-count) *count*)
(define (incr-count) (set! *count* (+ *count* 1)))
(lambda (x)
(syntax-case x ()
((m) (begin (incr-count) (datum->syntax #'m (get-count))))))))
Edited to add: In Racket, you can also do this:
(begin-for-syntax
(define *count* 0)
(define (get-count) *count*)
(define (incr-count) (set! *count* (+ *count* 1))))
(define-syntax m
(lambda (x)
(syntax-case x ()
((m) (begin (incr-count) (datum->syntax #'m (get-count)))))))
But I don't think R6RS has anything that corresponds to begin-for-syntax.