Tuple projection draft

Author: johnyob

rfcs/tuple_projections.md

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+# Tuple Projections 
+
+## Overview 
+
+This RFC proposes a quality-of-life improvement to OCaml's tuples, adding
+support for labeled and unlabeled tuple projections. 
+
+## Proposed change 
+
+The idea is to allow users to directly project elements from a tuple using
+labels or indicies, as opposed to patterns: 
+
+```ocaml 
+# let x = (~tuple:42, ~proj:1337, "is", 'c', 00, 1);;
+val x : (tuple: int * proj:int * string * char * int * int) = 
+  (~tuple:42, ~proj:1337, "is", 'c', 0, 1)
+# x.tuple;; 
+- : int = 42
+# x.3;;
+- : char = 'c'
+```
+
+Here, we're able to project out of a 6-tuple (containing both labeled and
+unlabeled components) simply by writing `x.j` (for an index `j`) or `x.l` (for
+a label `l`). Tuple indices are 0-indexed, to match the existing indexing
+convention for arrays, lists, etc. 
+
+This is useful for a couple reasons: 
+- Conciseness: avoids unnecessary boilerplate pattern matches / projection
+  functions e.g. `Tuple2.fst, Tuple3.fst, ...`. Additionally, with the advent
+  of labeled tuples, such functions are less useful[^1].
+- Clarity: `x.1` is more readable than `let _, x1, _ = x in ...`
+- Parity with records: complements record field projection and aligns tuple
+  uses with other ML-like languages.
+
+Occurrences of explicit pattern matching for tuple projection are 
+reasonably frequent where tuple projections could otherwise be used 
+(found by doing a quick Sherlocode[^2]):
+- ~6k uses of projections on pairs, 
+- ~1.15k for triples,
+- ~210 for quadruples. 
+
+
+## Previous work 
+
+Many other strongly-typed languages support built-in tuple projections.
+
+### SML 
+
+SML models tuples as records with integer field names (1-indexed), so projection uses
+record selection syntax:
+```sml 
+> val x = (1, "hi", 42);;
+val x = (1, "hi", 42): int * string * int
+> val y = #1 x;;
+val y = 1: int
+> val z = #3 x 
+val z = 42: int
+```
+
+### Rust 
+
+Rust supports tuple projections (0-indexed) for ordinary tuples (and tuple structs): 
+```rust 
+let x = (42, "is", 'c'); 
+let y = x.0; // 42
+let z = x.1; // "is"
+
+struct Point(i32, i32);
+let p = Point(3, 4);
+let x_coord = p.0; // 3
+```
+
+Record structs also use the same syntax for projection: 
+```rust 
+struct Point { x : i32, y : i32 };
+let p = Point { x = 3, y = 4 }; 
+let x_coord = p.x; // 3 
+```
+
+
+### Swift 
+
+Swift supports tuple projections via both positional indicies and labels (as in this proposal): 
+```swift 
+import Foundation 
+
+let x = (tuple: 42, proj: 1337, "is", 'c', 00, 1); 
+
+print(x.tuple) // 42
+print(x.5) // 1
+```
+
+## Implementation 
+
+An experimental implementation is available at [PR 14257](https://github.com/ocaml/ocaml/pull/14257).
+
+### Parsetree changes 
+
+Given the syntax for labeled tuple projection is overloaded with record field
+projection, i.e. there is no syntactic distinction between the projections in: 
+```ocaml 
+let x = { foo = 1; bar = 2 } in x.foo;;
+```
+and 
+```ocaml 
+let x = ~foo:1, ~bar:2 in x.foo;;
+```
+
+
+The proposed parsetree additions will represent all projections using `Pexp_field`:
+```ocaml
+type field = 
+  | Pfield_record_or_tuple_label of Longident.t loc 
+  | Pfield_tuple_index of int loc
+
+and expression_desc = 
+  ...
+  | Pexp_field of expression * field 
+  ... 
+```
+
+
+### Typechecking
+
+While typechecking, when encountering a field projection in expressions, 
+
+- If the field is a tuple index `j`, type as a unlabeled tuple projection. 
+
+  Check to see whether the expected type is known (principally known, if in `-principal` mode). 
+  Then:
+  * If the type is not known: raise an error stating that the projection is ambiguous. 
+  * If the type is known to be `(?l0:ty0 * ... * tyj * ... * ?ln:tyn)`: type the projection as `tyj` 
+
+- If the field is a record or tuple label `l`. 
+
+  Check to see whether the expected type is known: 
+  - If the type is not known: typecheck the projection `e.l` as a record projection 
+  - If the type is known to be `(ty0, ..., tyn) t`: ditto 
+  - If the type is known to be `(?l0:ty0 * ... * l:tyl * ... * ?ln:tyn)`: type the projection 
+    as `tyl`. 
+
+## Considerations
+
+### Limitations of type-based disambiguation
+
+OCaml's current type-based disambiguation mechanism is relatively weak. As a result, 
+many of the patterns that tuple projections are intended to replace would be ill-typed under 
+today's implementation. For instance: 
+```ocaml 
+# List.map (fun x -> x.1) [42, "Hello"; 1337, "World"];;
+Error: The type of the tuple expression is ambiguous.
+       Could not determine the type of the tuple projection.
+```
+
+That said, this limitation does not arise from the feature itself, but from the
+weaknesses in OCaml's type propagation. Improving type propagation (separately)
+would benefit not only tuple projections, but other features that rely on
+type-based disambiguation (e.g. constructors and record fields). As such, we
+argue that tuple projections should not be rejected on this point alone, and
+that the broader issues of type propagation and disambiguation be addressed
+separately. 
+
+### Syntactic overloading
+
+This proposal reuses the existing projection syntax `e.l` for both record
+fields and labeled tuples. The primary motivator behind this is to avoid
+introducing new operators and keeps projection syntax uniform. 
+
+The downside is that it increases reliance on type-based disambiguation. 
+
+### Diagnostic quality of error messages 
+
+Type errors surrounding unknown fields will need to be refined. 
+In particular, when the compiler defaults a labeled projection to a record 
+field (even though it might also have been a labeled tuple projection), 
+the diagnostic report ought to make this clear. 
+
+Otherwise, programs like the following may yield cryptic messages:
+```ocaml 
+# let is_ill_typed_due_to_defaults x = 
+    let y = x.tuple_label_a in 
+    ignore (x : (tuple_label_a:int * string * bool)); 
+    (y, x.2) 
+Error: Unbound record field `tuple_label_a`
+```
+
+A clearer diagnostic could be: 
+```
+Error: The field `tuple_label_a` is unknown. 
+       The projection `x.tuple_label_a` was interpreted as a record field, 
+       but no such record field exists. 
+
+Hint: Did you mean to project from a labeled tuple instead? 
+      If so, add an annotation to disambiguate the projection. 
+```
+
+Other problematic examples include conflicts with existing records:
+```ocaml 
+# type discombobulating_record = { tuple_label_a : int };;
+type discombobulating_record = { tuple_label_a : int }
+# let is_ill_typed_due_to_defaults x = 
+    let y = x.tuple_label_a in 
+    ignore (x : (tuple_label_a:int * string * bool)); 
+    (y, x.2) 
+Error: The value `x` has type `discombobulating_record` but an expression was
+        expected of type `tuple_label_a:int * string * bool`
+```
+Here the error conflates record and tuple typing, which is misleading. 
+A more informative report could combine a warning with the final error:
+```ocaml 
+Warning: The projection `x.tuple_label_a` could refer either to a record field 
+  or a labeled tuple component. It was resolved as a record field of 
+  `discombobulating_record`.
+  Please disambiguate if this is wrong.
+Error: The value `x` has type `discombobulating_record` but an expression
+       was expected of type 
+        `tuple_label_a:int * string * bool`
+```
+
+
+### Row-polymorphic tuples 
+
+Unlabeled tuple projections can naturally (and efficiently) be typed using row polymorphism, in 
+the same way object fields are typed today: 
+```ocaml 
+# let snd x = x.2;;
+val snd : 'a * 'b * .. -> 'b
+```
+
+This generalises to tuples of arbitrary arity (and is strictly more powerful than this proposal). 
+However, extending the same mechanism to labeled tuples is significantly more difficult (without incurring 
+runtime overhead or using monomorphisation). 
+
+From a language design perspective, though, we would ideally want the typing of projections 
+to behave uniformly across both labeled and unlabeled tuples. Moreover, the typing behaviour 
+discussed in this RFC is compatible with this use of row polymorphism. 
+
+
+### Non-principal defaults
+
+For type-based disambiguation, OCaml usually implements a 'default' behaviour when 
+the type is unknown. For instance, in record field / variant overloading, if the type is 
+not known to be a nominal type `(ty0, ..., tyn) t`, the lexically-closed matching record field / variant 
+is used. 
+
+We could have a similar default rule for tuple projections `e.j`, if the type is unknown: type `e` as `min 2 (j + 1)`-ary tuple. 
+
+[^1]: Without relying on coercions between labeled and unlabeled tuples, as in [PR 14180](https://github.com/ocaml/ocaml/pull/14180). And even then, it would be shorter to write a pattern than the types for `:>`.
+
+[^2]: The following family of patterns were used to derive estimates: [`fun (\w\+, _)`](https://sherlocode.com/?q=fun%20(%5Cw%5C%2B%2C%20_)), [`fun (\w\+, _, _)`](https://sherlocode.com/?q=fun%20(%5Cw%5C%2B%2C%20_%2C%20_)), [`fun (\w\+, _, _, _)`](https://sherlocode.com/?q=fun%20(%5Cw%5C%2B%2C%20_%2C%20_%2C%20_)), [`fun (_, \w\+)`](https://sherlocode.com/?q=fun%20(_%2C%20%5Cw%5C%2B)), [`fun (_, \w\+, _)`](https://sherlocode.com/?q=fun%20(_%2C%20%5Cw%5C%2B%2C%20_)), [`fun (_, \w\+, _, _)`](https://sherlocode.com/?q=fun%20(_%2C%20%5Cw%5C%2B%2C%20_%2C%20_)), etc.