Deprecate {{with}}

Summary

The {{with}} helper has always had slightly confusing conditional semantics, this was one of the motivators for introducing the easier to understand {{let}} helper. Now that {{let}} exists, the remaining use case for using {{with}} is for its unique conditional semantics. These conditional semantics can be cleanly represented with a combination of {{let}} and {{if}} statements so we should deprecate {{with}}.

Motivation

The difference between {{let}} and {{with}} is with how they handle conditional arguments. The {{let}} helper's block content is always rendered, regardless of its parameters. In contrast, {{with}} only renders its main block when the first position parameter is truthy. For example:

{{#with "Alex" as |value|}}
  {{value}} is truthy
{{else}}
  The first positional param was falsy
{{/with}}

Will render "[Alex] is truthy".

{{#with false as |value|}}
  {{value}} is truthy
{{else}}
  The first positional param was falsy
{{/with}}

Will render "The first positional param was falsy".

The conditional arguments behavior of {{with}} can easily be replicated using a combination of {{let}} and {{if}} in a way that's easily readable:

{{#let "Alex" as |value|}}
  {{#if value}}
    {{value}} is truthy
  {{else}}
    The first positional param was falsy
  {{/if}}
{{/let}}

Detailed design

We'll create an AST transform in packages/ember-template-compiler which will emit a deprecation warning for all uses of {{with}}. The deprecation warning will be:

Using `{{with}}` is deprecated, please use `{{let}}` instead.

This message will link to the following deprecation details which aim to give clear guidance on how to migrate to using {{let}}, {{if}} and {{else}} in different combinations:

---

The use of {{with}} has been deprecated, you should replace it with either {{let}} or a combination of {{let}}, {{if}} and {{else}}:

If you always want the block to render, replace {{with}} with {{let}} directly:

Before:

{{#with (hash name="Ben" age=4) as |person|}}
  Hi {{person.name}}, you are {{person.age}} years old.
{{/with}}

After:

{{#let (hash name="Ben" age=4) as |person|}}
  Hi {{person.name}}, you are {{person.age}} years old.
{{/let}}

If you want to render a block conditionally, use a combination of {{let}} and {{if}}:

Before:

{{#with user.posts as |blogPosts|}}
  There are {{blogPosts.length}} blog posts
{{/with}}

After:

{{#let user.posts as |blogPosts|}}
  {{#if blogPosts}}
    There are {{blogPosts.length}} blog posts
  {{/if}}
{{/let}}

If you want to render a block conditionally, and otherwise render an alternative block, use a combination of {{let}}, {{if}} and {{else}}:

Before:

{{#with user.posts as |blogPosts|}}
  There are {{blogPosts.length}} blog posts
{{else}}
  There are no blog posts
{{/with}}

After:

{{#let user.posts as |blogPosts|}}
  {{#if blogPosts}}
    There are {{blogPosts.length}} blog posts
  {{else}}
    There are no blog posts
  {{/if}}
{{/let}}

---

For people on older versions of Ember that support {{let}}, we'll create an ember-template-lint rule that they can use to prevent the use of {{with}}.

We'll also create a codemod which will assist people when migrating from {{with}} to {{let}}.

How we teach this

We'll mentiton the deprecation in an Ember point release blog post.

As mentioned above, the deprecation message will contain a link to clear guidelines on how to migrate to using {{let}}.

There is nothing to remove from the Ember.js Guides since we already teach only the use of {{let}}.

Drawbacks

This adds a little churn to Ember's API.

Alternatives

We could leave {{with}} as is. I don't believe that this is a good option as the name {{with}} is confusing.

We could deprecate {{with}} and introduce {{if-let}} in Ember core. This RFC originally made that exact proposal, I was strongly persuaded of the lack of need for {{if-let}} by @tcjr.

We could deprecate {{with}} and introduce {{if-let}} in an addon instead of Ember core.

Unresolved questions

(none)