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minima.l

Minimal lisp interpreter in about 350 lines of OCaml code. Heavily inspired by Picolisp.

Example

#
# Define the Fibonacci function
#
(def fib (N)
  (?: (<= N 1)
    N
    (+ (fib (- N 1)) (fib (- N 2)))
    ))

#
# Call the Fibonacci function
#
(prinl "Result: " (fib 30))

Language

The encoding is expected to be UTF-8.

# This is a comment

Types

  • List : ( ... )
  • Number: positive and negative 64-bit integer
  • Symbol: character string
  • String: "-delimited character string
  • Function: the function type as defined by def or λ
  • Internal: internal function definition, inaccessible to the user

Constants

  • T: stands for true
  • NIL: the empty list, also stands for false
  • _: wildcard, used to disregard patterns during deconstruction

Symbols

The interpreter uses symbols to address lisp types. Symbols can be created or altered using the def and setq functions. Internals are pre-mapped into the symbol world and can be overridden by the user.

Evaluation

  • Numbers and strings evaluate to themselves
  • Symbols evaluate to their values
  • Lists evaluate as function call

When a symbol is evaluated, its value is returned. In the case of internal functions (car, cdr, ...), the special Internal type is returned and displayed as <internal>:

: car
-> <car>

Lambda functions

Lambda functions are defined using the λ keyword (codepoint 0x3BB). Invocation of λ is similar to def:

: ((λ (X Y)(+ X Y)) 1 1)
-> 2

: ((λ x (map (λ (n)(+ n 1)) x)) '(1 2 3 4))
-> (2 3 4 5)

Closures

Function definitions carry a symbol closure. The closure contains the value of the function's symbols not present in the argument list such as they were resolved at the definition site.

Currying

Function can be curried:

: ((λ (a b) (+ a b)) 1)
-> (λ (b) (+ a b))
: (@ 10)
-> 11

Curryring is available for all user-defined functions as well as for some internal functions such as +, -, and, cons, and so on. Many shortcuts can therefore been defined as such:

: (setq =0 (= 0))
-> (λ (a) (= 0 a))
: (=0 0)
-> T
: (=0 1)
-> Nil

Recursion

Function defined using def can be recursive, i.e. call themselves. When functions are defined, symbols with their name are not resolved in their closure and are resolved in the symbol domain instead. Same goes when multiple functions are defined using def, allowing mutual recursion between functions. However, there is a caveat: since the function symbols are resolved dynamically, redefining these symbols will lead to undefined behavior.

Argument assignation

Assignation of arguments in either def of lamda functions support deconstruction. For instance, with def:

: (def sum3 ((a b c)) (+ (+ a b) c))
-> sum3
: (sum3 (list 1 2 3))
-> 6

Or with a lambda:

: (setq data '(("hello" . 1) ("world" . 2)))
-> (("hello" . 1) ("world" . 2))
: (foldl (λ (acc (_ . v))(+ acc v)) 0 data)
-> 3

Functions

A quoted (') argument means that it is evaluated by the function.

Basics

args
(args)

When executed in a script, return the list of command line arguments passed to the script.

#!/usr/bin/env mnml

(println args)
def
(def sym args [str] prg ...)

Define a function with arguments args and body prg and associate it with the symbol sym. An optional str can be specified as a documentation string and is ignored by the interpreter.

: (def add (x y) (+ x y))
-> add

Function defined with the def keyword are simply lambda functions assigned to symbol. Indeed, the following expression are strictly equivalent:

: (def add (a b) (+ a b))
-> add
: (setq add (λ (a b) (+ a b)))
-> (λ (a b) (+ a b))

Multiple functions can be defined at the same time. This is especially useful when functions are mutually recursive:

: (def
    a0 (n) (?: (< n 10) (b0 (+ n 1)) (cons 'a0 n))
    b0 (n) (?: (< n 10) (a0 (+ n 1)) (cons 'b0 n)))
-> b
: (a0 1)
-> (b0 . 10)
eval
(eval any)

Evaluate any.

: (eval '(+ 1 1))
-> 2
let
(let lst . prg)

Evaluate prg within the context of the bind list lst. The bind list has the following format:

((any . 'any)(any . 'any)...)

For each element in the bind list, the cdr is evaluated and bound to its car using the argument assignation process described above.

: (let ((a . 1)(b . 2)) (println a b))
1
2
-> 2
lift
(lift 'fn)

Lift a function object into a lambda definition list. The function's closure is lost in the process.

: (def add (a b) (+ a b))
-> add
: add
-> λ ((a b) (+ a b))
: (lift @)
-> (λ (a b) (+ a b))
load
(load . str)

Load the lisp file pointed by str. On success, load returns the result of the last evaluated operation in the file. Otherwise, NIL is returned.

: (load "examples/fibonacci.l")
-> 832040

If the path is prefixed by @lib, load will look for the file in the library directory of the installation prefix.

: (load "@lib/list.l")
-> assoc
quote
(quote . any)

Quote any. The form 'any is a syntactic shortcut for this function.

: (quote . a)
-> a
setq
(setq sym 'any [sym 'any] ...)

Associate any with the symbol sym. Multiple associations can take place at once.

: (setq A (+ 1 2) B (* A 2))
-> 6
: A
-> 3
: B
-> 6
sym
(sym . str)

Make a symbol of str.

: (sym . "+")
-> +
: (eval (sym . "+"))
-> <+>
: ((sym . "+") 1 1)
-> 2

List processing

car
(car 'lst)

Return the head of a list.

: (car (1 2 3 4))
-> 1
cdr
(cdr 'lst)

Return the tail of a list.

: (cdr (1 2 3 4))
-> (2 3 4)
conc
(conc 'any ...)

Concatenate multiple lists into one.

cons
(cons 'any ...)

Construct a new list cell using the first argument for car and the remaining arguments for cdr.

: (cons 1 2)
-> (1 . 2)
: (cons 1 2 3)
-> (1 2 . 3)
: (cons 1 (cons 2 3))
-> (1 2 . 3)
list
(list 'any ...)

Create a list with any arguments.

: (list)
-> (NIL)
: (list (+ 1 1) 3 "a")
-> (2 3 "a")
: (list (setq A 1) 2 (+ A 2))
-> (1 2 3)

Predicates

(nil? 'any)
(num? 'any)
(str? 'any)
(sym? 'any)
(lst? 'any)
(fun? 'any)

Respectively test if any is a number, a string, a symbol, a list, or an internal function. Return T on success, NIL otherwise.

Logic

(and 'any 'any)
(or 'any 'any)
(not 'any)

Perform the respective logic evaluation. In each case, any must evaluate to T or NIL.

Flow control

?
(? 'any prg)

When any evaluates to T, return the evaluation of prg. Return NIL otherwise.

: (def test (v) (? (> v 10) (* v 2)))
-> test
: (test 5)
-> NIL 
: (test 20)
-> 40
?!
(?! 'any prg)

When any evaluates to NIL, return the evaluation of prg. Return NIL otherwise.

: (def test (v) (?! (> v 10) (* v 2)))
-> test
: (test 5)
-> 10 
: (test 20)
-> NIL
case
(case 'any (any . prg) (any . prg) ...)

Compare any with the car of the remaining arguments and return the evaluation of the first positive match. The default or catch all case is written using the special value _ as car.

Order is important. If multiple match exist, the first one is evaluated. If _ is placed before a valid match, _ is evaluated.

: (def test (v) (case v ("hello" . "world") ("foo" . "bar") (_ . "unknown")))
-> test
: (test "hello")
-> "world"
: (test "foo")
-> "bar"
: (test "bonjour")
-> "unknown"

Also, using the wildcard symbol _, structural matches can be performed. For instance, here is an example that filters a list of lambdas based on their arity:

: (setq lambdas
    '(("inc" `(λ (x) (+ x 2)))
      ("add" `(λ (x y) (+ x y)))
      ("acc" `(λ (l) (foldl + 10 l)))
      ))
-> (("inc" ((x) (+ x 2))) ("add" ((x y) (+ x y))) ("acc" ((l) (foldl + 10 l))))

: (def filter (lamdas)
    (foldr (λ ((_ fn) acc)
             (case fn
               (((_  ) _) (cons @ acc))
               (((_ _) _) acc)
               (_         acc)
               ))
           lambdas
           ()
           ))
-> filter

: (filter lambdas)
-> ([(x) (+ x 2)] [(l) (foldl + 10 l)])

: (map (λ (l)(l 1)) (filter lambdas))
-> (3 11)
?:
(?: 'any prg1 prg2)

If any evaluates to T, return the evaluation of prg1. Return the evaluation of prg2 otherwise.

: (def test (v) (?: (> v 10) (* v 2) (* v 3)))
-> test
: (test 5)
-> 15
: (test 15)
-> 30
prog
(prog prg1 prg2 ...)

Evaluate prg1, prg2, ..., in sequence and return the last evaluation.

: (prog (+ 1 1) (+ 2 2))
-> 4
while
(while 'any prg)

Execute prg while any is not NIL.

: (while (read) (prinl @))
(+ 1 1)
<+>11
("hello" "world")
helloworld
NIL
-> ("hello" "world")

Exceptions

catch
(catch prg (any . 'any) (any . 'any) ...)

Catch exceptions thrown in prg. The exception is compared to the car of the catch clause. In case of a match, the cdr of the clause is run.

: (catch
    (throw "hello")
    ("hello" . "world")
    ("foo" . (println "bar")))
-> "world"
throw
(throw 'any)

Trow an exception with argument any.

Comparisons

=
(= 'num 'num)
(= 'str 'str)
(= 'lst 'lst)

Structural equality.

<>
(<> 'num 'num)
(<> 'str 'str)
(<> 'lst 'lst)

Structural inequality. Inverse of =.

ge, gt, le, lt
(ge 'num 'num)
(gt 'num 'num)
(le 'num 'num)
(lt 'num 'num)

Arithmetics

(add 'num 'num)
(div 'num 'num)
(mul 'num 'num)
(sub 'num 'num)

String operations

join
(join 'sep 'lst)

Join the list of strings lst into a single string using sep as a delimiter. When sep evaluates to NIL instead of a string, no separator is used.

: (join ":" ("a" "b" "c"))
-> "a:b:c"
split
(split 'sep 'str)

Split str into multiple strings using sep as a delimiter. When sep evaluates to NIL instead of a string, individual characters are returned.

: (split " " "hello world")
-> ("hello" "world")

I/O

flush
(flush)

Flush the current output channel.

json
(json 'sym 'str)

Parse JSON input. sym can either be file or string. This function piggies back on Yojson.

: (json 'string "{ \"hello\": \"world\" }")
-> (("hello" . "world"))
in
(in 'any . prg)

Create a new input channel context and evaluate prg within that context. The previous context is restored after the evaluation. When the first argument evaluates to NIL, the context uses stdin. When the argument evaluates to a string, in assumes the string contains a file path and tries to open that file.

: (in NIL (read))
1
-> 1
out
(out 'any . prg)

Create a new output channel context and evaluate prg within that context. The previous context is restored after the evaluation. When the first argument evaluates to NIL, the context uses stdout. When the argument evaluates to a string, out assumes the string contains a file path and tries to open that file.

If the file does not exist, it is created. If the file exists, it is truncated. If the file path is prepended with a + the file must exist and data will be appended to it.

: (setq DATA '(("Name"   "Age" "City"  )
               ("Alex"   "32" "London" )
               ("John"   "17" "Chicago")
               ("Marc"   "25" "Lyon"   )
               ("Sophie" "29" "Nice"   )
               ))
-> (("Name" "Age" "City") ("Alex" "32" "London") ("John" "17" "Chicago") ("Marc" "25" "Lyon") ("Sophie" "29" "Nice"))

: (out "result.csv"
    (iter (λ (t)(prinl (join ", " t))) DATA))
-> ("Sophie" "29" "Nice")
$ cat result.csv
Name, Age, City
Alex, 32, London
John, 17, Chicago
Marc, 25, Lyon
Sophie, 29, Nice
prin
(prin 'any ...)

Print the string representation of any. When multiple arguments are printed, no separator is used. The last argument is returned after evaluation.

: (prin "hello, " "world!")
hello, world!-> "world!"
prinl
(prinl 'any ...)

Calls prin and appends a new line.

: (prinl "hello, " "world!")
hello, world!
-> "world!"
print
(print 'any ...)

Print the lisp representation of any. When multiple arguments are printed, a space separator is used. The last argument is returned after evaluation.

: (print 'a 'b (1 2 3) +)
a b (1 2 3) <+>-> <+>
println
(println 'any ...)

Calls print and appends a new line.

: (print 'a 'b (1 2 3) +)
a b (1 2 3) <+>
-> <+>
read
(read)

Read one lisp token from the current input channel.

: (read)
(1 2 3)
-> (1 2 3)

System

env
(env 'any ['any'])

If any evaluates as a string, return the value of the environment variable with that string. If a second any argument is provided and evaluates as a string, set the value of the environment variable to that string and return the previous value.

: (env "PATH")
-> "/usr/bin:/bin"
: (env "PATH" (join ":" (list "/usr/local/bin" (env "PATH"))))
-> "/usr/bin:/bin"
: (env "PATH")
-> "/usr/local/bin:/usr/bin:/bin"
quit
(quit)

Terminate the top-level evaluation.

Debug

closure

Print the closure of a function definition.

: (def add (a b) (+ a b))
-> add
: (setq +1 (add 1))
-> (add (b) (+ a b))
: (closure +1)
-> (("a" . 1) ("+" . <+>))
trace

Toggle tracing.

Appreciation

Tokens of appreciation are gladly accepted in the form of virtual coffee.

License

ISC. See LICENSE.md.