Started working on 'try' statement

ucl/builtins.go

@@ -770,6 +770,71 @@ func ifBuiltin(ctx context.Context, args macroArgs) (object, error) {
 	return nil, errors.New("malformed if-elif-else")
 }
 
+func tryBuiltin(ctx context.Context, args macroArgs) (object, error) {
+	if args.nargs() < 1 {
+		return nil, errors.New("need at least 1 arguments")
+	}
+
+	res, err := args.evalBlock(ctx, 0, nil, false)
+	if err == nil {
+		return res, nil
+	}
+
+	args.shift(1)
+	for args.identIs(ctx, 0, "catch") {
+		args.shift(1)
+
+		if args.nargs() < 1 {
+			return nil, errors.New("need at least 1 arguments")
+		}
+
+		res, err := args.evalBlock(ctx, 0, nil, false)
+		if err == nil {
+			return res, nil
+		}
+
+		args.shift(2)
+	}
+
+	// TODO: handle uncaught error
+
+	return nil, nil
+
+	/*
+		if guard, err := args.evalArg(ctx, 0); err == nil && isTruthy(guard) {
+			return args.evalBlock(ctx, 1, nil, false)
+		} else if err != nil {
+			return nil, err
+		}
+
+		args.shift(2)
+		for args.identIs(ctx, 0, "elif") {
+			args.shift(1)
+
+			if args.nargs() < 2 {
+				return nil, errors.New("need at least 2 arguments")
+			}
+
+			if guard, err := args.evalArg(ctx, 0); err == nil && isTruthy(guard) {
+				return args.evalBlock(ctx, 1, nil, false)
+			} else if err != nil {
+				return nil, err
+			}
+
+			args.shift(2)
+		}
+
+		if args.identIs(ctx, 0, "else") && args.nargs() > 1 {
+			return args.evalBlock(ctx, 1, nil, false)
+		} else if args.nargs() == 0 {
+			// no elif or else
+			return nil, nil
+		}
+
+		return nil, errors.New("malformed if-elif-else")
+	*/
+}
+
 func foreachBuiltin(ctx context.Context, args macroArgs) (object, error) {
 	var (
 		items    object

ucl/inst.go

@@ -88,6 +88,7 @@ func New(opts ...InstOption) *Inst {
 	rootEC.addMacro("if", macroFunc(ifBuiltin))
 	rootEC.addMacro("foreach", macroFunc(foreachBuiltin))
 	rootEC.addMacro("proc", macroFunc(procBuiltin))
+	rootEC.addMacro("try", macroFunc(tryBuiltin))
 
 	inst := &Inst{
 		out:    os.Stdout,

ucl/testbuiltins_test.go

@@ -3,6 +3,7 @@ package ucl
 import (
 	"bytes"
 	"context"
+	"errors"
 	"fmt"
 	"strings"
 	"testing"
@@ -36,6 +37,13 @@ func WithTestBuiltin() InstOption {
 			return listObject(args.args), nil
 		}))
 
+		i.rootEC.addCmd("error", invokableFunc(func(ctx context.Context, args invocationArgs) (object, error) {
+			if len(args.args) == 0 {
+				return nil, errors.New("an error occurred")
+			}
+			return nil, errors.New(args.args[0].String())
+		}))
+
 		i.rootEC.addCmd("joinpipe", invokableFunc(func(ctx context.Context, args invocationArgs) (object, error) {
 			sb := strings.Builder{}
 
@@ -188,6 +196,52 @@ func TestBuiltins_If(t *testing.T) {
 	}
 }
 
+func TestBuiltins_Try(t *testing.T) {
+	tests := []struct {
+		desc string
+		expr string
+		want string
+	}{
+		{desc: "single try - successful", expr: `
+			try {
+				echo "good"
+			}
+			echo "after"`, want: "good\nafter\n(nil)\n"},
+		{desc: "single try - unsuccessful", expr: `
+			try {
+				error "bang"
+			}
+			echo "after"`, want: "after\n(nil)\n"},
+		{desc: "try with catch - successful", expr: `
+			try {
+				echo "good"
+			} catch {
+				echo "something happened"
+			}
+			echo "after"`, want: "good\nafter\n(nil)\n"},
+		{desc: "try with catch - unsuccessful", expr: `
+			try {
+				error "bang"
+			} catch {
+				echo "something happened"
+			}
+			echo "after"`, want: "something happened\nafter\n(nil)\n"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.desc, func(t *testing.T) {
+			ctx := context.Background()
+			outW := bytes.NewBuffer(nil)
+
+			inst := New(WithOut(outW), WithTestBuiltin())
+			err := EvalAndDisplay(ctx, inst, tt.expr)
+
+			assert.NoError(t, err)
+			assert.Equal(t, tt.want, outW.String())
+		})
+	}
+}
+
 func TestBuiltins_ForEach(t *testing.T) {
 	tests := []struct {
 		desc string