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114 lines
2.5 KiB
Go
114 lines
2.5 KiB
Go
package main
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import (
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"fmt"
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"reflect"
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)
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type special func(*environment, ...interface{}) (interface{}, error)
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type nargsInvalidError struct {
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expected int
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received int
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name string
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}
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func (n nargsInvalidError) Error() string {
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return fmt.Sprintf(`received %d arguments in *%v*, expected %d`,
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n.received, n.name, n.expected)
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}
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// defines the built-in "define" construct. e.g.:
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//
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// (define x 5)
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//
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// would create the symbol "x" and set its value to 5.
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func define(env *environment, args ...interface{}) (interface{}, error) {
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if len(args) != 2 {
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return nil, nargsInvalidError{2, len(args), "define"}
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}
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s, ok := args[0].(symbol)
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if !ok {
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return nil, fmt.Errorf(`first argument to *define* must be symbol, received %v`, reflect.TypeOf(args[0]))
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}
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v, err := eval(args[1], env)
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if err != nil {
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return nil, err
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}
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env.set(s, v)
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return nil, nil
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}
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// defines the built-in "quote" construct. e.g.:
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//
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// (quote (1 2 3))
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//
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// would evaluate to the list (1 2 3). That is, quote is a function of arity 1
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// that is effectively a no-op; the input value is not evaluated, which
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// prevents evaluation of the first element of the list, in this case 1.
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func quote(_ *environment, args ...interface{}) (interface{}, error) {
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if len(args) != 1 {
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return nil, nargsInvalidError{1, len(args), "quote"}
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}
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return args[0], nil
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}
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// defines the built-in "if" contruct. e.g.:
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//
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// (if #t "foo" "bar")
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//
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// would evaluate to "foo", while the following:
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//
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// (if #f "foo" "bar")
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//
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// would evaluate to "bar"
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func _if(env *environment, args ...interface{}) (interface{}, error) {
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if len(args) != 3 {
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return nil, nargsInvalidError{3, len(args), "if"}
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}
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v, err := eval(args[0], env)
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if err != nil {
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return nil, err
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}
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if b, ok := v.(bool); ok && !b {
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return eval(args[2], env)
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}
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return eval(args[1], env)
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}
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// defines the built-in "set" construct, which is used to set the value of an
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// existing symbol in the provided environment. e.g.:
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//
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// (set! x 5)
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//
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// would set the symbol x to the value 5, if and only if the symbol x was
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// previously defined.
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func set(env *environment, args ...interface{}) (interface{}, error) {
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if len(args) != 2 {
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return nil, nargsInvalidError{2, len(args), "set!"}
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}
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s, ok := args[0].(symbol)
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if !ok {
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return nil, fmt.Errorf(`first argument to *set!* must be symbol, received %v`, reflect.TypeOf(args[0]))
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}
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if !env.defined(s) {
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return nil, fmt.Errorf(`cannot *set!* undefined symbol %v`, s)
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}
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v, err := eval(args[1], env)
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if err != nil {
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return nil, err
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}
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env.set(s, v)
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return nil, nil
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}
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