Clarification: nil vs empty string

[scottrigby]

Hello!

While reviewing recent charts PRs (example: helm/charts#2004), I see this interesting pattern:

{{- if .Values.persistence.storageClass }}
{{- if (eq "-" .Values.persistence.storageClass) }}
  storageClassName: ""
{{- else }}
  storageClassName: "{{ .Values.persistence.storageClass }}"
{{- end }}
{{- end }}

The explanation is:

  ## mariadb data Persistent Volume Storage Class
  ## If defined, storageClassName: <storageClass>
  ## If set to "-", storageClassName: "", which disables dynamic provisioning
  ## If undefined (the default) or set to null, no storageClassName spec is
  ##   set, choosing the default provisioner.  (gp2 on AWS, standard on
  ##   GKE, AWS & OpenStack)
  ##
  # storageClass: "-"

My question was, since the template-rendered YAML will either be a storage class name, an empty string (to disable dynamic provisioning), or not set at all, can't we just do this instead?

  ## mariadb data Persistent Volume Storage Class
  ## If defined, storageClassName: <storageClass>
  ## If set to empty string "", this disables dynamic provisioning
  ## If undefined (the default) or set to null, no storageClassName spec is
  ##   set, choosing the default provisioner.  (gp2 on AWS, standard on
  ##   GKE, AWS & OpenStack)
  ##
  # storageClass: ""

Then the template logic could be simplified to:

{{- if .Values.persistence.storageClass }}
  storageClassName: "{{ .Values.persistence.storageClass }}"
{{- end }}

But this would only work if an empty string is different from not set (nil) when checking if .Values.some.value in go template. Can someone help clarify this for me, or point to the relevant part of the docs? I seem to not be able to find this info.

Thanks!

[scottrigby]

OK, I found the relevant helm issue: helm/helm#2600

But just checking here, is there really no way for sprig to differentiate the difference between nil and "" (empty string) in a template? If not, would that be a desirable PR?

[technosophos]

Wow. That issue actually helped me nail down an issue I was seeing in a local PVC! Ha!

Okay, so an {{ if $value }} will check for both "" and nil, considering either one to be an "empty value". There is also the empty function, which essentially behaves the same way.

It might be nice to be able to test whether something is nil and possibly empty ({{ if isNil $foo }}). I don't think we have a way of checking that. Maybe eq $foo nil would work... I've never tried.

Does that answer your question, or am I misunderstanding.

[mattfarina]

@technosophos If something is a string, isn't the initialized value on a struct "" instead of nil? How would you get the nil state without the template engine complaining about the missing field?

I imagine I'm missing something which is why I ask.

[jkroepke]

Found a solution {{ kindIs "invalid" $value }}

[mattfarina]

@scottrigby @technosophos Here's an example of the previous comment... https://play.golang.org/p/9yJW5iGT4qa

When I looked at this in the past I'd not thought of this. Thanks @jkroepke

[scottrigby]

Love it 😄

[thesuperzapper]

Found a solution {{ kindIs "invalid" $value }}

@jkroepke While this solution might work, I imagine there are other cases where the "kind" of something might be "invalid" without it being nil.

I think better option is to use a direct equality check against nil:

{{ eq $value nil }}

PS: the reason that the kind of nil equals "invalid" is actually because nil does not have a type in Go, because it is a "predeclared identifier".