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|
package main
import (
"fmt"
"strconv"
"strings"
"text/scanner"
"github.com/alecthomas/kong"
"github.com/alecthomas/participle/v2"
"github.com/alecthomas/participle/v2/lexer"
"github.com/alecthomas/repr"
)
type operatorPrec struct{ Left, Right int }
var operatorPrecs = map[string]operatorPrec{
"+": {1, 1},
"-": {1, 1},
"*": {3, 2},
"/": {5, 4},
"%": {7, 6},
}
type (
Expr interface{ expr() }
ExprIdent struct{ Name string }
ExprString struct{ Value string }
ExprNumber struct{ Value float64 }
ExprParens struct{ Sub Expr }
ExprUnary struct {
Op string
Sub Expr
}
ExprBinary struct {
Lhs Expr
Op string
Rhs Expr
}
)
func (ExprIdent) expr() {}
func (ExprString) expr() {}
func (ExprNumber) expr() {}
func (ExprParens) expr() {}
func (ExprUnary) expr() {}
func (ExprBinary) expr() {}
func parseExprAny(lex *lexer.PeekingLexer) (Expr, error) { return parseExprPrec(lex, 0) }
func parseExprAtom(lex *lexer.PeekingLexer) (Expr, error) {
switch peek := lex.Peek(); {
case peek.Type == scanner.Ident:
return ExprIdent{lex.Next().Value}, nil
case peek.Type == scanner.String:
val, err := strconv.Unquote(lex.Next().Value)
if err != nil {
return nil, err
}
return ExprString{val}, nil
case peek.Type == scanner.Int || peek.Type == scanner.Float:
val, err := strconv.ParseFloat(lex.Next().Value, 64)
if err != nil {
return nil, err
}
return ExprNumber{val}, nil
case peek.Value == "(":
_ = lex.Next()
inner, err := parseExprAny(lex)
if err != nil {
return nil, err
}
if lex.Peek().Value != ")" {
return nil, fmt.Errorf("expected closing ')'")
}
_ = lex.Next()
return ExprParens{inner}, nil
default:
return nil, participle.NextMatch
}
}
func parseExprPrec(lex *lexer.PeekingLexer, minPrec int) (Expr, error) {
var lhs Expr
if peeked := lex.Peek(); peeked.Value == "-" || peeked.Value == "!" {
op := lex.Next().Value
atom, err := parseExprAtom(lex)
if err != nil {
return nil, err
}
lhs = ExprUnary{op, atom}
} else {
atom, err := parseExprAtom(lex)
if err != nil {
return nil, err
}
lhs = atom
}
for {
peek := lex.Peek()
prec, isOp := operatorPrecs[peek.Value]
if !isOp || prec.Left < minPrec {
break
}
op := lex.Next().Value
rhs, err := parseExprPrec(lex, prec.Right)
if err != nil {
return nil, err
}
lhs = ExprBinary{lhs, op, rhs}
}
return lhs, nil
}
type Expression struct {
X Expr `@@`
}
var parser = participle.MustBuild[Expression](participle.ParseTypeWith(parseExprAny))
func main() {
var cli struct {
Expr []string `arg required help:"Expression to parse."`
}
ctx := kong.Parse(&cli)
expr, err := parser.ParseString("", strings.Join(cli.Expr, " "))
ctx.FatalIfErrorf(err)
repr.Println(expr)
}
|
