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229 lines
5.4 KiB
Go

package main
import (
"errors"
"fmt"
"reflect"
)
// type special is a callable outside of the normal execution workflow. That
// is, a special receives its arguments unevaluated, unlike lambdas or builtin,
// both of whose arguments are evaluated upon invocation.
type special func(*environment, ...interface{}) (interface{}, error)
// type arityError is used to store information related to arity errors. That
// is, the invocation of a callable with the wrong number of arguments.
type arityError struct {
expected int
received int
name string
}
func (n arityError) Error() string {
return fmt.Sprintf(`received %d arguments in *%v*, expected %d`,
n.received, n.name, n.expected)
}
// helper function to check the arity of incoming arguments for a function.
// Also accepts the case that the args slice is nil.
func checkArity(arity int, args []interface{}, name string) error {
if args == nil {
if arity == 0 {
return nil
}
return arityError{arity, 0, name}
}
if len(args) != arity {
return arityError{arity, len(args), name}
}
return nil
}
// defines the built-in "define" construct. e.g.:
//
// (define x 5)
//
// would create the symbol "x" and set its value to 5.
func define(env *environment, args ...interface{}) (interface{}, error) {
if err := checkArity(2, args, "define"); err != nil {
return nil, err
}
s, ok := args[0].(symbol)
if !ok {
return nil, fmt.Errorf(`first argument to *define* must be symbol, received %v`, reflect.TypeOf(args[0]))
}
v, err := eval(args[1], env)
if err != nil {
return nil, err
}
env.set(s, v)
return nil, nil
}
// defines the built-in "quote" construct. e.g.:
//
// (quote (1 2 3))
//
// would evaluate to the list (1 2 3). That is, quote is a function of arity 1
// that is effectively a no-op; the input value is not evaluated, which
// prevents evaluation of the first element of the list, in this case 1.
func quote(_ *environment, args ...interface{}) (interface{}, error) {
if err := checkArity(1, args, "quote"); err != nil {
return nil, err
}
return args[0], nil
}
// turns an arbitrary lisp value into a boolean. Apparently the sematics of
// this in lisp are that everything except false is true? Seems weird to me,
// but ok.
func booleanize(v interface{}) bool {
if b, ok := v.(bool); ok {
return b
}
return true
}
// defines the built-in "if" contruct. e.g.:
//
// (if #t "foo" "bar")
//
// would evaluate to "foo", while the following:
//
// (if #f "foo" "bar")
//
// would evaluate to "bar"
func _if(env *environment, args ...interface{}) (interface{}, error) {
if err := checkArity(3, args, "if"); err != nil {
return nil, err
}
v, err := eval(args[0], env)
if err != nil {
return nil, err
}
if booleanize(v) {
return eval(args[1], env)
}
return eval(args[2], env)
}
// defines the built-in "set!" construct, which is used to set the value of an
// existing symbol in the provided environment. e.g.:
//
// (set! x 5)
//
// would set the symbol x to the value 5, if and only if the symbol x was
// previously defined.
func set(env *environment, args ...interface{}) (interface{}, error) {
if err := checkArity(2, args, "set!"); err != nil {
return nil, err
}
s, ok := args[0].(symbol)
if !ok {
return nil, fmt.Errorf(`first argument to *set!* must be symbol, received %v`, reflect.TypeOf(args[0]))
}
if !env.defined(s) {
return nil, fmt.Errorf(`cannot *set!* undefined symbol %v`, s)
}
v, err := eval(args[1], env)
if err != nil {
return nil, err
}
env.set(s, v)
return nil, nil
}
type lambda struct {
env *environment
arglabels []symbol
body sexp
}
func (l lambda) call(env *environment, rawArgs []interface{}) (interface{}, error) {
debugPrint("call lambda")
args := make([]interface{}, 0, len(rawArgs))
for _, raw := range rawArgs {
v, err := eval(raw, env)
if err != nil {
return nil, err
}
args = append(args, v)
}
if len(args) != len(l.arglabels) {
return nil, errors.New("parity error")
}
for i := range args {
l.env.set(l.arglabels[i], args[i])
}
return eval(l.body, l.env)
}
// defines the built-in lambda construct. e.g.:
//
// (lambda (x) (* x x))
//
// would evaluate to a lambda that, when executed, squares its input.
func mklambda(env *environment, args ...interface{}) (interface{}, error) {
debugPrint("mklambda")
if err := checkArity(2, args, "lambda"); err != nil {
return nil, err
}
params, ok := args[0].(sexp)
if !ok {
return nil, fmt.Errorf(`first argument to *lambda* must be sexp, received %v`, reflect.TypeOf(args[0]))
}
arglabels := make([]symbol, 0, len(params))
for _, v := range params {
s, ok := v.(symbol)
if !ok {
return nil, fmt.Errorf(`lambda args must all be symbols; received invalid %v`, reflect.TypeOf(v))
}
arglabels = append(arglabels, s)
}
body, ok := args[1].(sexp)
if !ok {
return nil, fmt.Errorf(`second argument to *lambda* must be sexp, received %v`, reflect.TypeOf(args[1]))
}
return lambda{env, arglabels, body}, nil
}
// defines the built-in "begin" construct. A "begin" statement evaluates each
// of its inputs, and returns the value of the evaluation of the last
// statement. E.g.:
//
// (begin (+ 1 1) (* 2 2) (+ 3 3))
//
// would evaluate to 6.
func begin(env *environment, args ...interface{}) (interface{}, error) {
debugPrint("begin")
var err error
var v interface{}
for _, arg := range args {
v, err = eval(arg, env)
if err != nil {
return nil, err
}
}
return v, nil
}