gohugoio/hugo · error

complement needs at least two arguments

Error message

complement needs at least two arguments

What it means

Thrown by the Complement template function when fewer than 2 arguments are provided (complement.go:33). Complement treats all args except the last as 'subtrahend' sets and the last as the 'universe' to filter; with <2 args there is no universe to subtract from, so it is rejected immediately.

Solutions

1. Provide at least two arguments: a subtraction set and a universe, e.g. {{ $all | complement $exclude }}.
2. If you only have one set and want it unchanged, do not call complement at all.
3. Verify the pipe: {{ .Pages | complement $last4 }} yields args = [$last4, .Pages].

Example fix

// before
{{ complement $exclude }}
// after
{{ .Pages | complement $exclude }}

{{/* complement needs >=2 args; pipe form supplies them */}}
{{ .Pages | complement $exclude }}

When it happens

Trigger: Calling {{ complement }} (no args) or {{ complement $only }} (one arg). In templates the pipe form {{ .Pages | complement $last4 }} supplies two args; forgetting the piped left-hand operand or calling with a literal single arg triggers it.

Common situations: Refactoring a pipeline so .Pages no longer flows in; using complement as a unary filter by mistake; removing the 'universe' argument.

Related errors

• invalid number of arguments to Seq
• arguments to complement must be slices or arrays
• both count and seq must be provided
• can't evaluate the array by no match argument or more than…
• cannot index slice/array with nil

AI-assisted analysis of gohugoio/hugo@52c9bd7908 (2026-08-09). Data as JSON: /api/errors/126e4be5d53d0913. Report an issue: GitHub.

Appendix: source

Thrown at tpl/collections/complement.go:34

import (
	"errors"
	"fmt"
	"reflect"

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

// Complement gives the elements in the last element of ls that are not in
// any of the others.
//
// All elements of ls must be slices or arrays of comparable types.
//
// The reasoning behind this rather clumsy API is so we can do this in the templates:
//
//	{{ $c := .Pages | complement $last4 }}
func (ns *Namespace) Complement(ls ...any) (any, error) {
	if len(ls) < 2 {
		return nil, errors.New("complement needs at least two arguments")
	}

	universe := ls[len(ls)-1]
	as := ls[:len(ls)-1]

	aset, err := collectIdentities(as...)
	if err != nil {
		return nil, err
	}

	v := reflect.ValueOf(universe)
	switch v.Kind() {
	case reflect.Array, reflect.Slice:
		sl := reflect.MakeSlice(v.Type(), 0, 0)
		for i := range v.Len() {
			ev, _ := hreflect.Indirect(v.Index(i))
			if _, found := aset[normalize(ev)]; !found {
				sl = reflect.Append(sl, ev)

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