LispInSmallPieces/chapter3/interpreter.coffee

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{listToString, listToVector, pairp, cons, car, cdr, caar, cddr, cdar, cadr, caadr, cadar, caddr, nilp, nil, setcdr, metacadr} = require "cons-lists/lists"
print = require "../chapter1/print"
# Debugging tool.
{inspect} = require "util"
env_init = nil
env_global = env_init
ntype = (node) -> car node
nvalu = (node) -> cadr node
definitial = (name, value = nil) ->
env_global = (cons (cons name, value), env_global)
name
defprimitive = (name, nativ, arity) ->
definitial name, ((args) ->
vmargs = listToVector(args)
if (vmargs.length == arity)
nativ.apply null, vmargs
else
throw "Incorrect arity")
the_false_value = (cons "false", "boolean")
definitial "#t", true
definitial "#f", the_false_value
definitial "nil", nil
definitial "foo"
definitial "bar"
definitial "fib"
definitial "fact"
defpredicate = (name, nativ, arity) ->
defprimitive name, ((a, b) -> if nativ.call(null, a, b) then true else the_false_value), arity
defprimitive "cons", cons, 2
defprimitive "car", car, 2
defprimitive "set-cdr!", setcdr, 2
defprimitive "+", ((a, b) -> a + b), 2
defprimitive "*", ((a, b) -> a * b), 2
defprimitive "-", ((a, b) -> a - b), 2
defprimitive "/", ((a, b) -> a / b), 2
defpredicate "lt", ((a, b) -> a < b), 2
defpredicate "eq?", ((a, b) -> a == b), 2
extend = (env, variables, values) ->
if (pairp variables)
if (pairp values)
(cons (cons (car variables), (car values)),
(extend env, (cdr variables), (cdr values)))
else
throw "Too few values"
else if (nilp variables)
if (nilp values) then env else throw "Too many values"
else
if (symbolp variables)
(cons (cons variables, values), env)
else
nil
make_function = (variables, body, env) ->
(values) -> eprogn body, (extend env, variables, values)
invoke = (fn, args) ->
(fn args)
# Takes a list of nodes and calls evaluate on each one, returning the
# last one as the value of the total expression. In this example, we
# are hard-coding what ought to be a macro, namely the threading
# macros, "->"
eprogn = (exps, env) ->
if (pairp exps)
if pairp (cdr exps)
evaluate (car exps), env
eprogn (cdr exps), env
else
evaluate (car exps), env
else
nil
evlis = (exps, env) ->
if (pairp exps)
(cons (evaluate (car exps), env), (evlis (cdr exps), env))
else
nil
lookup = (id, env) ->
if (pairp env)
if (caar env) == id
cdar env
else
lookup id, (cdr env)
else
nil
update = (id, env, value) ->
if (pairp env)
if (caar env) == id
setcdr value, (car env)
value
else
update id, (cdr env), value
else
nil
# This really ought to be the only place where the AST meets the
# interpreter core. I can't help but think that this design precludes
# pluggable interpreter core.
astSymbolsToLispSymbols = (node) ->
return nil if nilp node
throw "Not a list of variable names" if not (ntype(node) is 'list')
handler = (node) ->
return nil if nilp node
cons (nvalu car node), (handler cdr node)
handler(nvalu node)
# Takes an AST node and evaluates it and its contents. A node may be
# ("list" (... contents ...)) or ("number" 42) or ("symbol" x), etc.
cadddr = metacadr('cadddr')
evaluate = (e, env) ->
[type, exp] = [(ntype e), (nvalu e)]
if type == "symbol"
return lookup exp, env
else if type in ["number", "string", "boolean", "vector"]
return exp
else if type == "list"
head = car exp
if (ntype head) == 'symbol'
switch (nvalu head)
when "quote" then cdr exp
when "if"
unless (evaluate (cadr exp), env) == the_false_value
evaluate (caddr exp), env
else
evaluate (cadddr exp), env
when "begin" then eprogn (cdr exp), env
when "set!" then update (nvalu cadr exp), env, (evaluate (caddr exp), env)
when "lambda" then make_function (astSymbolsToLispSymbols cadr exp), (cddr exp), env
else invoke (evaluate (car exp), env), (evlis (cdr exp), env)
else
invoke (evaluate (car exp), env), (evlis (cdr exp), env)
else
throw new Error("Can't handle a #{type}")
module.exports = (c) -> evaluate c, env_global