What causes "can't apply myself (no turtles allowed)" in gohugoio/hugo?

Restructure nested mapping with `range` instead: loop the outer slice and call `apply` on each inner slice.

can't apply myself (no turtles allowed)

Error message

can't apply myself (no turtles allowed)

What it means

`collections.Apply` maps a named template function over each element of a slice. Hugo explicitly forbids `fname` being `"apply"` itself, because letting apply invoke apply would allow unbounded recursive self-application ("turtles all the way down"); it rejects the call up front.

Source

Thrown at tpl/collections/apply.go:33

import (
	"context"
	"errors"
	"fmt"
	"reflect"
	"strings"

	"github.com/gohugoio/hugo/common/hreflect"
)

// Apply takes an array or slice c and returns a new slice with the function fname applied over it.
func (ns *Namespace) Apply(ctx context.Context, c any, fname string, args ...any) (any, error) {
	if c == nil {
		return make([]any, 0), nil
	}

	if fname == "apply" {
		return nil, errors.New("can't apply myself (no turtles allowed)")
	}

	seqv := reflect.ValueOf(c)
	seqv, isNil := hreflect.Indirect(seqv)
	if isNil {
		return nil, errors.New("can't iterate over a nil value")
	}

	fnv, found := ns.lookupFunc(ctx, fname)
	if !found {
		return nil, errors.New("can't find function " + fname)
	}

	switch seqv.Kind() {
	case reflect.Array, reflect.Slice:
		r := make([]any, seqv.Len())
		for i := range seqv.Len() {
			vv := seqv.Index(i)

View on GitHub ↗ (pinned to 8a468df065)

Solutions

Restructure nested mapping with `range` instead: loop the outer slice and call `apply` on each inner slice.
Use a different function name — apply any function except apply itself.
If the name is computed, validate it before the call.

Example fix

<!-- before -->
{{ $r := apply $seqOfSeqs "apply" "." "upper" "." }}
<!-- after -->
{{ $r := slice }}
{{ range $seqOfSeqs }}
  {{ $r = $r | append (apply . "upper" ".") }}
{{ end }}

Defensive patterns

Strategy: validation

Validate before calling

{{/* never pass "apply" as the function name to apply */}}
{{ $upper := apply $seq "strings.ToUpper" "." }}

Prevention

apply cannot invoke itself — never use "apply" as the function-name argument
For nested per-element transformations, use range with an inner apply-free expression, or chain multiple apply calls sequentially
Restructure recursive-looking logic into partials called with `partial` instead

When it happens

Trigger: Calling `{{ apply $seq "apply" ... }}` — passing the literal function name "apply" as the function to apply.

Common situations: Attempting to nest apply calls to map a function over a slice of slices; generating the function name dynamically and it resolving to "apply".

Related errors

AI-assisted analysis of gohugoio/hugo@8a468df065 (2026-07-31). Data as JSON: /data/errors/6914ef185238a331.json. Report an issue: GitHub ↗.