scpt/scpt.go

// Package scpt provides an interpreter for my simple applescript-like
// scripting language
package scpt

import (
	"errors"
	"fmt"
	"github.com/alecthomas/participle"
	"github.com/antonmedv/expr"
	"io"
	"reflect"
	"strings"
)

// Vars stores any variables set during script runtime
var Vars = map[string]interface{}{}

// FuncMap is a map of strings mapped to suitable script functions
type FuncMap map[string]func(map[string]interface{}) (interface{}, error)

// Funcs stores the functions allowed for use in a script
var Funcs = FuncMap{
	"display-dialog":  displayDialog,
	"do-shell-script": doShellScript,
}

// AddFuncs adds all functions from the provided FuncMap into
// the Funcs variable
func AddFuncs(fnMap FuncMap) {
	// Add each function to Funcs
	for name, fn := range fnMap {
		Funcs[name] = fn
	}
}

// AddVars adds all functions from the provided map into
// the Vars variable
func AddVars(varMap map[string]interface{}) {
	for name, val := range varMap {
		// Add each variable to Vars
		Vars[name] = val
	}
}

// Parse uses participle to parse a script from r into a new AST
func Parse(r io.Reader) (*AST, error) {
	// Build parser from empty AST struct
	parser, err := participle.Build(&AST{})
	if err != nil {
		return nil, err
	}
	// Create new empty AST struct
	ast := &AST{}
	// Parse script from provided reader into ast
	err = parser.Parse(r, ast)
	if err != nil {
		return nil, err
	}
	// Return filled AST struct
	return ast, nil
}

// ParseValue parses a Value struct into a go value
func ParseValue(val *Value) (interface{}, error) {
	// Determine which value was provided and return it
	if val.String != nil {
		// Return unquoted string
		return strings.Trim(*val.String, `"`), nil
	} else if val.Bool != nil {
		// Return dereferenced Bool converted to bool
		return bool(*val.Bool), nil
	} else if val.Float != nil {
		// Return dereferenced float
		return *val.Float, nil
	} else if val.Integer != nil {
		// Return dereferenced integer
		return *val.Integer, nil
	} else if val.SubCmd != nil {
		// Return reference to subcommand
		return val.SubCmd, nil
	} else if val.VarVal != nil {
		// Return value of provided key
		return Vars[*val.VarVal], nil
	} else if val.Expr != nil {
		// Parse value of left side of expression
		left, _ := callIfFunc(ParseValue(val.Expr.Left))
		// If value is string, requote
		if isStr(left) {
			left = requoteStr(left.(string))
		}
		// Create new nil string
		var right string
		// For every right segment
		for _, segment := range val.Expr.RightSegs {
			// Parse value of right segment, calling it if it is a function
			rVal, _ := callIfFunc(ParseValue(segment.Right))
			// If value is string, requote
			if isStr(rVal) {
				rVal = requoteStr(rVal.(string))
			}
			// Append right segment to right string
			right = right + fmt.Sprintf(
				" %s %v",
				segment.Op,
				rVal,
			)
		}
		// Create string expression from segments and operator
		exp := fmt.Sprintf(
			"%v %s",
			left,
			right,
		)
		// Compile string expression
		program, err := expr.Compile(strings.ReplaceAll(exp, "^", "**"))
		if err != nil {
			return nil, err
		}
		// Run expression
		out, err := expr.Run(program, Vars)
		if err != nil {
			return nil, err
		}
		// Return expression output value
		return out, nil
	}
	return nil, nil
}

// Add quotes to an unquoted string
func requoteStr(s string) string {
	// Return quoted string
	return `"` + s + `"`
}

// Check if i is a string
func isStr(i interface{}) bool {
	// if type of input is string, return true
	if reflect.TypeOf(i).String() == "string" {
		return true
	}
	return false
}

// Call val if it is a function, otherwise pass through return values
func callIfFunc(val interface{}, err error) (interface{}, error) {
	if err != nil {
		return nil, err
	}
	// If val is a pointer to a FuncCall
	if IsFuncCall(val) {
		// Pass through return values of function call
		return CallFunction(val.(*FuncCall))
	}
	// Return given value
	return val, nil
}

// UnwrapArgs takes a slice of Arg structs and returns a map
// storing the argument name and its value. If the argument has
// no name, its key will be an empty string
func UnwrapArgs(args []*Arg) (map[string]interface{}, error) {
	// Create new empty map of strings to any type
	argMap := map[string]interface{}{}
	// For each argument
	for _, arg := range args {
		// Parse value into interface{}
		val, err := ParseValue(arg.Value)
		if err != nil {
			return nil, err
		}
		// If value is function call
		if IsFuncCall(val) {
			// Call function, setting its value as the argument's value
			argMap[arg.Key], err = CallFunction(val.(*FuncCall))
			if err != nil {
				return nil, err
			}
			// Skip further code and start next loop
			continue
		}
		// Set argument value to parsed value
		argMap[arg.Key] = val
	}
	// Return map of arguments
	return argMap, nil
}

// IsFuncCall checks if val is a FuncCall struct
func IsFuncCall(val interface{}) bool {
	// If type of val is a pointer to FuncCall, return true
	if reflect.TypeOf(val) == reflect.TypeOf(&FuncCall{}) {
		return true
	}
	return false
}

// CallFunction executes a given function call in the form of
// a FuncCall struct
func CallFunction(call *FuncCall) (interface{}, error) {
	// Unwrap provided arguments
	argMap, err := UnwrapArgs(call.Args)
	if err != nil {
		return nil, err
	}
	// Attempt to get function from Funcs map
	fn, ok := Funcs[call.Name]
	if !ok {
		return nil, errors.New("no such function: " + call.Name)
	}
	// Return value received from function
	return fn(argMap)
}