== HaScheme A Lazy dialect of Scheme, embedded in Scheme [[toc:]] === Description ''Scheme demonstrates that a very small number of rules for forming expressions, with no restrictions on how they are composed, suffice to form a practical and efficient programming language that is flexible enough to support most of the major programming paradigms in use today.'' — ''Revisedⁿ Reports on the Algorithmic Language Scheme (n ≥ 3, 1986–)'' HaScheme is a library that implements a subset of the R7RS's libraries in a lazy way, using {{force}} and {{delay-force}}. When HaScheme is used by itself, it is a lazy functional programming language with the same syntax as Scheme, embedded within Scheme. HaScheme is implicitly lazy, and only requires strictness annotations, not laziness annotations. Scheme procedures can be easily wrapped to be used in HaScheme, and Scheme can call {{force}} on HaScheme promises to obtain values from HaScheme. For instance, the following {{map}} implementation is both valid HaScheme (importing {{(hascheme base)}}) and Scheme (importing {{(scheme base)}}): (define (map f list) (if (null? list) '() (cons (f (car list)) (map f (cdr list))))) This procedure in HaScheme will return a promise. The function's body is only executed if, when attempting to {{force}} a promise containing the map, the contents of the map are needed for a computation. Since HaScheme is lazy, one can write infinite lists with it: (let ((N (let loop ((i 0)) (cons (! i) (loop (+ i 1)))))) (force (list-ref (map square N) 100))) ; => 100000 This code snippet will run in a constant amount of space. (The procedure {{!}} is a strictness annotation to avoid thunk buildup. Since HaScheme is emedded within a call-by-value language, the strictness annotation will always force {{i}} at every iteration of the loop, even if neither part of the pair is accessed.) HaScheme does not support continuations, parameters, exceptions, or multiple value returns. === Author Peter McGoron === Repository https://software.mcgoron.com/hascheme/ === Requirements * [[/egg/r7rs|r7rs]] === API The HaScheme library namespace is structured in a similar way to R7RS-large's namespace, except it does not export any procedure that is side-effecting. ==== {{(hascheme base)}} Exports everything from {{(scheme base)}} as lazy procedures, except * {{call/cc}}, {{dynamic-wind}}, {{values}}, {{call-with-values}} * Any procedure ending with {{!}} * Anything to do with ports, except for {{eof-object}} and {{eof-object?}} * {{make-parameter}}, {{parameterize}} * Anything involving exceptions, except for {{error}}, {{error-object?}}, {{error-object-message}}, {{error-object-irritants}} * {{do}} * {{floor/}}, {{truncate/}}, and {{exact-integer-sqrt}} are exported, but return lists. The functions act as one would expect they would act. Forcing {{(car x)}} forces {{x}}, but forcing {{(cons x y)}} does not force {{x}} or {{y}}. Syntax that introduces procedures, like {{lambda}}, {{define}}, and named {{let}} make lazy procedures. Lazy procedures return a promise that when forced, force the body of the procedure. Note that control syntax like {{if}} and {{cond}} are still syntax. They can be implemented as functions, but then they will have subtly different behavior when it comes to explicitly forcing values. String, bytevector, and number constructors are always eager. List and vector constructors are lazy. This library also exports {{seq}}. (seq x ... y) Forces all arguments except its last argument. Returns its last argument unchanged. ==== {{(hascheme case-lambda)}} Exports {{case-lambda}}, which creates lazy procedures. ==== {{(hascheme char)}} Exports lazy procedure versions of {{(scheme char)}}. ==== {{(hascheme complex)}} Exports lazy procedure versions of {{(scheme complex)}}. ==== {{(hascheme cxr)}} Exports lazy procedure versions of {{(hascheme cxr)}}. ==== {{(hascheme inexact)}} Exports lazy procedure versions of {{(hascheme inexact)}}. ==== {{(hascheme control)}} Exports procedure versions of control syntax. (if x y [z]) Force {{x}}. If true, return {{y}}. If not, return {{z}} (or an unspecified value). (cond [x y] ...) Force {{x}}. If true, return {{y}}. If not, continue on with the next pairs. If no pairs exist, return an unspecified value. (and x ...) Force {{x}}. If true, force the next value, and so on. If there are no more values, return {{#t}}. If not, return {{#f}}. (or x ...) Force {{x}}. If true, return {{x}}. Otherwise, force the next value, and so on. If there are no more values, return {{#f}}. (when pred . subsequents) Force {{pred}}. If true, run {{(apply seq subsequents)}}. Otherwise return an unspecified value. (unless pred . subsequents) Force {{pred}}. If false, run {{(apply seq subsequents)}}. Otherwise return an unspecified value. ==== {{(hascheme eager)}} Procedures and syntax forms for eager evaluation. (! x) Shorthand for {{(force x)}}. (define-wrappers-from-strict (head source) ...) Defines lazy-procedure variants of {{source}} that force all arguments. Head can either be {{(name formal ...)}} or {{name}}. (define-wrappers-for-lazy (head source) ...) Define lazy-procedure variants of {{source}} that pass all arguments to a constructor. Head is defined as in {{define-wrappers-from-strict}}. (define-binary-wrapper (wrapper source) ...) Defines {{wrapper}} as a non-strict n-ary comparison function. It will force the first two arguments to it, and if the comparison returns false, the procedure returns false and none of the other values are forced. Otherwise it will keep on forcing arguments until there are no more arguments or one of the comparisons fail. (let*! ((formal expr) ...) body ...) Forces each {{expr}}, binds the result to {{formal}}, and executes {{body ...}}. (let*-seq ((formal expr) ...) body ...) Returns a promise to force each {{expr}}, bind it to {{formal}}, and then execute {{body ...}}. This library also exports {{seq}}. === Misc. Information ==== How is it implemented? The implementation follows the formula of [[https://srfi.schemers.org/srfi-45/|SRFI-45]] with some changes for usability. *# All procedure bodies are implicitly wrapped in {{delay-force}}. *# Scheme procedures that require their values are, in addition, wrapped such that forcing their function forces the necessary arguments. A Scheme procedure that is a constructor does not force its arguments. *# Control flow forces the value of the test expression. Three critical things are needed to make HaScheme ergonomic: *# Promises are distinct from other types. *# Forcing a value returns the value. *# Forcing {{(delay-force x)}} is equivalent to a tail call to {{(force x)}}. Chicken does all three. There is nothing fancy going on here, just some (relatively) easy-to-understand {{syntax-rules}} macros. ==== Fun (or pain) with Laziness You need to be careful with lazy functions because they can cause space leaks. This is a problem in general with lazy languages [[https://wiki.haskell.org/Foldr_Foldl_Foldl%27|likein Haskell]]). Here is an example: (define (list-tail list n) (if (zero? n) list (list-tail (cdr list) (- n 1)))) Thunks will build up over time in the list, so it must be forced. (define (list-tail list n) (if (zero? n) list (list-tail (force (cdr list)) (- n 1)))) Note that {{n}} is never explicitly forced: it is implicitly forced by the control flow. The first code block has the attractive property that it operates the same way on finite lists in both Scheme and HaScheme, while the second one could differ in exotic cases (like promises that return promises). Instead of writing {{force}}, the operator {{!}} is used: (define (list-tail list n) (if (zero? n) list (list-tail (! (cdr list)) (- n 1)))) Now if one were to define {{!}} as the identity function in regular, call-by-value Scheme, then the block above will evaluate to the same value in both HaScheme and Scheme, in bounded space. Ok, now we have fixed our space leak issues. Right? Let's try another infinite list trick: a list of all natural numbers. (define naturals (list-tabulate +inf.0 (lambda (x) x))) (! (list-tail naturals 1000000000)) This also leaks! This is because the promises are making new cons cells, and storing them in {{naturals}}. We need to organize things to make sure the program can clean up. (! (list-tail (list-tabulate +inf.0 (lambda (x) x)) 1000000000)) This will run in bounded space. ==== Call-by-Need and Conditionals Since call-by-need will only execute a function when needed, conditional forms like {{if}} can be implemented as functions and not syntax. In fact, {{(hascheme control)}} implements {{if}}, {{and}}, {{or}}, and the {{cond}} as functions, meaning one can pass them around as values. For instance: (define (map f l) (cond ((null? l) '()) ((pair? l) (cons (f (car l)) (cdr l))) (else (error "not a list" l)))) implemented with {{(hascheme control)}} is (define (map f l) (cond (null? l) '() (pair? l) (cons f (car l) (cdr l)) #t (error "not a list" l))) Neat, right? Well, if we go to {{list-tail}} we have a problem: (define (list-tail list n) (if (zero? n) list (list-tail (! (cdr list)) (- n 1)))) Since {{if}} is now a function, Scheme (our call-by-value host language) will attempt to reduce {{(! (cdr list))}} every time, even when we don't need to. We could go back to syntactic if, or we could add some wrapper to the procedure. The {{seq}} function (named after the function in Haskell) takes {{n}} forms, forces the first {{n-1}}, and returns the {{n}}th form. (define (list-tail list n) (if* (zero? n) list (seq (cdr list) (list-tail (cdr list) (- n 1))))) ==== Multiple Values and Continuations HaScheme doesn't have {{call/cc}}. {{call/cc}} is not a function because it does not return, so that's strike one for inclusion in a pure language. Reified continuations make sense in a call-by-value language, because there is a definite evaluation order (outermost first), but a lazy language can execute any code at basically any time. A future implementation might be able to use SRFI-226's delimited control structures to implement continuations, because they are actual functions. Multiple values are specified as returning values to their continuation. Since HaScheme does not (conceptually) have continuations, multiple values have to be interpreted differently. But a bigger issue occurs because a promise is a single value. It cannot be decomposed into more values without forcing the promise. === Issues/Future Directions * Lightly tested. * I want to implement some purely functional data structures. === License Copyright 2025 Peter McGoron Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at [[http://www.apache.org/licenses/LICENSE-2.0]]. Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.