Add try

Updates #9737

Author: bep

tpl/internal/go_templates/texttemplate/hugo_template.go

@@ -15,6 +15,7 @@ package template
 
 import (
 	"context"
+	"fmt"
 	"io"
 	"reflect"
 
@@ -255,10 +256,32 @@ func (s *state) evalField(dot reflect.Value, fieldName string, node parse.Node,
 	panic("not reached")
 }
 
+// TryValue is what gets returned when using the "try" keyword.
+type TryValue struct {
+	// Value is the value returned by the function or method wrapped with "try".
+	// This will always be nil if Err is set.
+	Value any
+	// Err is the error returned by the function or method wrapped with "try".
+	// This will always be nil if Value is set.
+	Err error
+}
+
 // evalCall executes a function or method call. If it's a method, fun already has the receiver bound, so
 // it looks just like a function call. The arg list, if non-nil, includes (in the manner of the shell), arg[0]
 // as the function itself.
-func (s *state) evalCall(dot, fun reflect.Value, isBuiltin bool, node parse.Node, name string, args []parse.Node, final reflect.Value, first ...reflect.Value) reflect.Value {
+func (s *state) evalCall(dot, fun reflect.Value, isBuiltin bool, node parse.Node, name string, args []parse.Node, final reflect.Value, first ...reflect.Value) (val reflect.Value) {
+	// Added for Hugo.
+	if name == "try" {
+		defer func() {
+			if r := recover(); r != nil {
+				if err, ok := r.(error); ok {
+					val = reflect.ValueOf(TryValue{nil, err})
+				} else {
+					val = reflect.ValueOf(TryValue{nil, fmt.Errorf("%v", r)})
+				}
+			}
+		}()
+	}
 	if args != nil {
 		args = args[1:] // Zeroth arg is function name/node; not passed to function.
 	}
@@ -371,6 +394,11 @@ func (s *state) evalCall(dot, fun reflect.Value, isBuiltin bool, node parse.Node
 		s.helper.OnCalled(s.ctx, s.prep, name, argv, vv)
 	}
 
+	// Added for Hugo.
+	if name == "try" {
+		return reflect.ValueOf(TryValue{vv.Interface(), nil})
+	}
+
 	return vv
 }
 

tpl/partials/init.go

@@ -38,7 +38,7 @@ func init() {
 			},
 		)
 
-		// TODO(bep) we need the return to be a valid identifier, but
+		// TODO(bep) we need the return to be a valid identifiers, but
 		// should consider another way of adding it.
 		ns.AddMethodMapping(func() string { return "" },
 			[]string{"return"},

tpl/safe/init.go

@@ -70,6 +70,13 @@ func init() {
 			},
 		)
 
+		ns.AddMethodMapping(func(v any) (any, error) {
+			return v, nil
+		},
+			[]string{"try"},
+			[][2]string{},
+		)
+
 		return ns
 	}
 

tpl/templates/templates_integration_test.go

@@ -93,3 +93,37 @@ Home: true
 
 `)
 }
+
+func TestTry(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- config.toml --
+baseURL = 'http://example.com/'
+-- layouts/index.html --
+Home.
+{{ $g :=  try ("hello = \"Hello Hugo\"" | transform.Unmarshal)   }}
+{{ with $g.Err }}
+Err1: {{ . }}
+{{ else }}
+Value1: {{ $g.Value.hello | safeHTML }}|
+{{ end }}
+{{ $g :=  try ("hello != \"Hello Hugo\"" | transform.Unmarshal)   }}
+{{ with $g.Err }}
+Err2: {{ . | safeHTML }}
+{{ else }}
+Value2: {{ $g.Value.hello | safeHTML }}|
+{{ end }}
+Try upper: {{ (try ("hello" | upper)).Value }}
+Try printf: {{ (try (printf "hello %s" "world")).Value }}
+`
+
+	b := hugolib.Test(t, files)
+
+	b.AssertFileContent("public/index.html",
+		"Value1: Hello Hugo|",
+		"Err2: template: index.html:",
+		"Try upper: HELLO",
+		"Try printf: hello world",
+	)
+}