Sep 07, 2021
3 min read
Swifties REPL
introThis projects aims to demonstrate how to implement a custom Lisp REPL using Swifties .
<div class="snippet-clipboard-content position-relative" data-snippet-clipboard-copy-content="Swifties v5
Return evaluates completed forms,
(func fibrec [n:Int] [Int]
Swifties v5
Return evaluates completed forms,
(reset) clears the stack and Ctrl+D quits.
(func fibrec [n:Int] [Int]
(if (< $n 2) $n (+ (fibrec (-- $n)) (fibrec (- $n 2)))))
(fibrec 10)
[55]
quirks
The stack is directly exposed to user code, just like in Forth.
Primitives, Macros and Functions are called on reference outside of call forms.
Parens are used for calls only, brackets for lists of things.
stacksc swaps the specified number of items.
d drops the specified number of items.
s swaps the specified number of items.
stash replaces the the stack with its contents as a single item.
Stack literals are enclosed in brackets.
splat replaces the top item (which is required to be iterable) with its items.
bindingsValues may be bound to identifiers using let.
(let [x 35 y 7]
(+ $x $y))
[42]
functionsNew functions may be defined using func.
<div class="snippet-clipboard-content position-relative" data-snippet-clipboard-copy-content=" (func fibrec [n:Int] [Int]
(func fibrec [n:Int] [Int]
(if (< $n 2) $n (+ (fibrec (-- $n)) (fibrec (- $n 2)))))
(fibrec 10)
[55]
The same thing could be accomplished without bindings by manipulating the stack, if one was so inclined.
<div class="snippet-clipboard-content position-relative" data-snippet-clipboard-copy-content=" (func fibrecs [Int] [Int]
(func fibrecs [Int] [Int]
c (if (< _ 2) _ (do -- c fibrecs s -- fibrecs +)))
(fibrecs 10)
[55]
It also runs somewhat faster.
(bench 100 (fibrec 10))
[366]
(bench 100 (fibrecs 10))
[366 357]
The algorithm can definitely be improved, note that I had to change n from 10 to 50 to even get something worth measuring.
(func fibtail1 [n:Int a:Int b:Int] [Int]
(if (z? $n) $a (if (one? $n) $b (fibtail1 (-- $n) $b (+ $a $b)))))
(bench 100 (fibtail1 50 0 1))
[149]
Since the recursive call is in tail position, recall may be used to trigger tail call optimization.
(func fibtail2 [n:Int a:Int b:Int] [Int]
(if (z? $n) $a (if (one? $n) $b (recall (-- $n) $b (+ $a $b)))))
(bench 100 (fibtail2 50 0 1))
[149 115]
dots. may be used to shift arguments to the left of the target syntactically.
multimethodsFunctions are upgraded to multimethods as soon as multiple definitions share the same name. Multimethods delegate to the most specific applicable function, this makes them somewhat more expensive to call.
(func foo [] [String] "n/a")
(func foo [_:Int] [String] "Int")
(func foo [_:Any] [String] "Any")
(foo 42)
["Int"]
(foo t)
["Int" "Any"]
(foo)
["Int" "Any" "n/a"]
booleansEvery value has a boolean representation; most are true, but integers are false when zero, strings and stacks when empty etc.
and returns the first argument if false, else the last.
(and 0 t)
[0]
(and t 42)
[0 42]
or returns the first argument if true, else the last.
(or 42 f)
[42]
(or f 42)
[42 42]
notreturns t if the argument is false, else f.
(not 0)
[t]
(not 42)
[t f]
stringsString literals are enclosed in double quotes.
charactersCharacter literals are prefixed with \.
quotesForms may be quoted by prefixing with '.
'foo '(x y z)
['foo '(x y z)]
, may be used to splice values into quoted expressions.
(let [foo 42] '[,$foo])
['[42]]
Splicing multiple values is just as easy.
(let [foo [1 2 3]] '[,(splat $foo)])
['[1 2 3]]
pairsPairs may be formed using :.
iteratorsfor may be used to iterate any sequence.
: may be used to bind the current value within the loop body.
(for c:"foo" $c)
[\f \o \o]
map may be used to transform sequences.
(map &++ [1 2 3])
[Iter]
(splat _)
[2 3 4]
continuationsThe current continuation may be captured using suspend and evaluated using restore.
(suspend "was here") 42
[Cont(2) "was here"]
d (restore _)
[42]
todoGitHub
https://github.com/codr7/swifties-repl
