Add new basic helpers to Ember

text/0000-basic-template-helpers.md

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+- Start Date: 2018-10-13
+- RFC PR: (leave this empty)
+- Ember Issue: (leave this empty)
+
+# (RFC title goes here)
+
+## Summary
+
+Add new built-in template helpers to perform basic operations in templates, identical to the
+helpers in [ember-truth-helpers](https://github.com/jmurphyau/ember-truth-helpers)
+
+## Motivation
+
+It is a very common necessity to almost every Ember app to perform certain operations like compare
+by equality, negate a value, or perform boolean operations, and often the most convenient place to
+do it is in the templates.
+Because of that, `ember-truth-helpers` is probably the single most installed addon either directly
+or transitively in the entire ecosystem.
+
+The fact that this addon is so popular is very telling and Ember should consider to move into the
+core of the templating engine at least the most popular helpers.
+
+One of the shocking moments developers that are familiar with React, Vue or Angular have when trying Ember
+is that they cannot, at least out of the box, perform the most basic comparisons and boolean operations
+they are so used to in JSX or Vue/Angular templates.
+
+Furthermore, if the [RFC #367](https://github.com/emberjs/rfcs/pull/367) eventually gets merged, these
+helpers are the perfect candidates to create adoption friction in the community because of how
+pervasive its usage it in so many templates.
+
+Albeit less important, implementing these helpers in the core _could_ enable clever trickery in the
+Glimmer VM, like evaluating them at compile time when the arguments are constant.
+
+## Detailed design
+
+The process consists on deciding what helpers from `ember-truth-helpers` we consider the most important
+and move them into Ember.js itself or even the Glimmer VM, in a fully backwards compatible way.
+
+I propose to move to core at least:
+
+- `eq`
+- `not`
+- `and`
+- `or`
+- `gt` and `gte`
+- `lt` and `lte`
+
+Those helpers are very low lever and generally useful in both Ember and Glimmer.
+
+I propose to not move:
+
+- `is-array` (uses `Ember.isArray`)
+- `is-empty` (uses `Ember.isEmpty`)
+- `is-equal` (uses `Ember.isEqual`)
+- `xor`      (not very common)
+- `not-eq`   (can be expressed as `(not (eq a b)`)
+
+When this feature is implemented, we would update `ember-truth-helpers` to not ship the helpers that
+were moved into the core if the version of Ember is
+
+## How we teach this
+
+As any new API, will have to be documented in the guides with examples.
+
+## Drawbacks
+
+We are increasing the public API of the framework, and every line of code is a liability, although
+those helpers are extremely straightforward.
+
+## Alternatives
+
+An alternative path would be to include `ember-truth-helpers` in the default blueprint for apps an
+addons.
+
+## Unresolved questions
+
+The main unresolved question is what helpers we deem worthy of being moved into the core, and also
+if we want any other helpers not mentioned above.
+
+Other helpers that come to mind that _could_ also be worth adding:
+
+- `array` (there is an RFC for it: https://github.com/emberjs/rfcs/pull/318)
+- `add`, `subtract`, and other arithmetic operators.
+- `{{#await promise as |value|}}` to render a block conditionally if a promise resolves or fails.
+
+If there is interest in them, I advocate creating standalone RFCS for them.