form.go

//-----------------------------------------------------------------------------
// Copyright (c) 2022 Detlef Stern
//
// This file is part of sxpf.
//
// sxpf is licensed under the latest version of the EUPL // (European Union
// Public License). Please see file LICENSE.txt for your rights and obligations
// under this license.
//-----------------------------------------------------------------------------

package sxpf

import "fmt"

// Form is a value that can be called. Depending on IsSpecial, the arguments
// are evaluated or not before calling the form.
type Form interface {
	Value

	Call(Environment, []Value) (Value, error)
	IsSpecial() bool
}

// Builtin is a wrapper for a builtin function.
type Builtin struct {
	name     string
	fn       BuiltinFn
	minArity int
	maxArity int // if maxArity < minArity ==> maxArity is unlimited
	special  bool
}

// BuiltinFn is a builtin form that is implemented in Go.
type BuiltinFn func(Environment, []Value) (Value, error)

// NewBuiltin returns a new builtin form.
func NewBuiltin(name string, special bool, minArity, maxArity int, f BuiltinFn) *Builtin {
	return &Builtin{name, f, minArity, maxArity, special}
}

func (b *Builtin) Equal(other Value) bool {
	if b == nil || other == nil {
		return b == other
	}
	if o, ok := other.(*Builtin); ok {
		return b.name == o.name
	}
	return false
}

func (b *Builtin) String() string { return "#" + b.name }

func (b *Builtin) IsSpecial() bool { return b != nil && b.special }
func (b *Builtin) Name() string {
	if b == nil {
		return ""
	}
	return b.name
}

func (b *Builtin) Call(env Environment, args []Value) (Value, error) {
	if length := len(args); length < b.minArity {
		return nil, fmt.Errorf("not enough arguments (%d) for form %v (%d)", length, b.name, b.minArity)
	} else if b.minArity <= b.maxArity && b.maxArity < length {
		return nil, fmt.Errorf("too many arguments (%d) for form %v (%d)", length, b.name, b.maxArity)
	}
	return b.fn(env, args)
}

func (b *Builtin) GetValue() BuiltinFn {
	if b == nil {
		return nil
	}
	return b.fn
}