chaisql/internal/expr/functions/definition.go

package functions

import (
	"fmt"
	"strings"

	"github.com/chaisql/chai/internal/expr"
)

// variadicArity represents an unlimited number of arguments.
const variadicArity = -1

// A Definition transforms a list of expressions into a Function.
type Definition interface {
	Name() string
	String() string
	Function(...expr.Expr) (expr.Function, error)
}

// Definitions table holds a map of definition, indexed by their names.
type Definitions map[string]Definition

// GetFunc return a function definition by its package and name.
func GetFunc(fname string) (Definition, error) {
	def, ok := builtinFunctions[strings.ToLower(fname)]
	if !ok {
		return nil, fmt.Errorf("no such function: %q", fname)
	}
	return def, nil
}

// A definition is the most basic version of a function definition.
type definition struct {
	name          string
	arity         int
	constructorFn func(...expr.Expr) (expr.Function, error)
}

func (fd *definition) Name() string {
	return fd.name
}

func (fd *definition) Function(args ...expr.Expr) (expr.Function, error) {
	if fd.arity == variadicArity && len(args) == 0 {
		return nil, fmt.Errorf("%s() requires at least one argument", fd.name)
	}
	if fd.arity != variadicArity && (len(args) != fd.arity) {
		return nil, fmt.Errorf("%s() takes %d argument(s), not %d", fd.name, fd.arity, len(args))
	}
	return fd.constructorFn(args...)
}

func (fd *definition) String() string {
	arity := fd.arity
	if arity < 0 {
		arity = 0
	}
	args := make([]string, 0, arity)
	for i := 0; i < arity; i++ {
		args = append(args, fmt.Sprintf("arg%d", i+1))
	}
	return fmt.Sprintf("%s(%s)", fd.name, strings.Join(args, ", "))
}

func (fd *definition) Arity() int {
	return fd.arity
}