FLIP 210: propose an improvement to entitlement mapping syntax (#210)

* add flip

* Apply suggestions from code review

* clarify

cadence/20231016-entitlement-mapping-syntax.md

@@ -0,0 +1,59 @@
+---
+status: approved 
+flip: 210
+authors: Daniel Sainati ([email protected])
+sponsor: Daniel Sainati ([email protected])
+updated: 2023-10-16
+---
+
+# FLIP NNN: Improve Entitlement Mapping Syntax
+
+## Objective
+
+This proposes a syntax change to make entitlement-mapped fields more visually distinct from entitled fields.
+
+## Motivation
+
+User feedback has indicated that entitlement mappings are too easy to confuse with entitlements, 
+and since one of them has default behavior for public access and the other prevents it, 
+it can be difficult to reason about what fields and functions are publicly available in the presence
+of both entitled and mapped fields.
+
+For example, given some definitions:
+
+```
+entitlement X
+entitlement Y
+entitlement mapping M {
+    X -> Y
+}
+resource R {
+    access(X) let foo: T
+    access(M) let bar: T
+}
+```
+
+It is not immediately obvious that given a reference `r` of type `&R`, `r.foo` would fail statically but `r.bar` would succeed and produce a `&T` reference.
+
+## User Benefit
+
+This will make it easier for users to distinguish between entitled members and mapped members. 
+
+## Design Proposal
+
+Currently, an entitlement mapped field or an entitlement mapped reference is specified by using the
+name of a mapping (e.g. `M`) in an `access` or `auth` modifier (e.g. `access(M)` or `auth(M)`).  
+
+Instead, we would require the keyword `mapping` to be present in the modifiers to indicate
+that `M` is a mapping here, rather than an entitlement. 
+So the above modifiers would become `access(mapping M)` and `auth(mapping M)`
+
+### Drawbacks
+
+This makes the creation of entitlement mapped fields and functions 16 characters more verbose. I.e. a function
+that was previously declared as `access(M) fun foo(): auth(M) &T` would instead have to be declared as 
+`access(mapping M) fun foo(): auth(mapping M) &T`, which is more verbose. 
+
+### Alternatives Considered
+
+We have not discussed alternative syntactic changes, but are open to suggestions.