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fix(semantics): type sibling collection elements by their nearest shared supertype
Elements of a collection value whose types are siblings — a Truck and a Car, a
MassValue and a String — shared no listed type, so the value fell back to Anything
and a feature valued by it lost its common parent. sharedTypes now falls back to
the nearest supertypes every element conforms to, so (truck, car).?{…} is a Vehicle
collection and a body answering (x.mass, x.name) is a ScalarValue collection.
Co-Authored-By: jason.han <hanhuijun@gmail.com>
- **A collection operation's static type follows what its declaration hands through, not the element type of the collection.** `xs->collect { in x : C; x.mass }` and `xs.{ in x : C; x.mass }` are typed by the body's result (`MassValue`), a nested collect by its innermost body, a body answering a sequence by every element type and `xs->collect f` by the named function's result; `select`, `reject` and `selectOne` keep the elements of `xs`, `reduce` follows its reducer's result — and the element a one-element collection hands back unreduced, unless the collection is known to hold two or more, by its own multiplicity, one it inherits by redefinition, or a chain through such features; a collection holding one at most is never reduced, so its element alone is the result — and `forAll`/`exists` stay `Boolean`, each with the multiplicity the Kernel Function Library declares. A body whose result cannot be typed keeps the library's `Anything`. Value conformance, a feature's bound value, invocation arguments, trigger arguments and enumerated values are judged by the specialized type, so `accept when counts.{in n : Integer; n}` is refused where it was silent, and `when counts.{in n; n > 3}` is accepted where it was refused. A scalar literal a body writes out is as exact as one bound directly: `attribute i : Integer = xs.{ in x : C; 1.5 }` is refused, and a quantity it writes out is measured against the target's dimension: `attribute t : DurationValue = xs.{ in x : C; 5 [m] }` is refused. `xs.?{…}` binds and is typed as `xs->select {…}` is — `(v1, v2).?{ in v : Vehicle; true }` is a `Vehicle` collection, not the `Anything` the sequence is — and an element that is itself a collection value binds by the elements it holds, so `attribute i : Integer = xs.{ in x : C; xs.{ in y : C; 1.5 } }` is refused. A collection over `()` or a feature admitting no value keeps its declared type — `()->collect { in a : Integer; "s" }` is a `String` collection — but holds no element, so none is judged and its `reduce` takes nothing from an element it would hand back unreduced; so does one mapping every element to a `[0]` feature or function result — the multiplicity read through an alias, or from the feature or result a redefinition inherits it from — or any operation over such a collection, and an argument holding nothing is judged against no parameter type. An argument or constructor value that is a collection binds each element it holds on its own, so `Sail(vs.{ in v : Vehicle; (v, boat) })` and `new Fleet(vs.{ in v : Vehicle; 1.5 })` are refused by the element that does not bind where they were silent. A collection value binds as many values as it is known to hold, counted against the feature's multiplicity — `part b : Boat[1] = pair.items.{ in v : Vehicle; boat }` binds two — a `reduce` counting the one element it hands back unreduced or what its reducer yields over two or more, so `pair.items->reduce { in a : Vehicle; in b : Vehicle; (a, b) }` binds two and one over `()` none. A body reading the feature it values terminates as a self-referential argument does.
1
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- **A collection operation's static type follows what its declaration hands through, not the element type of the collection.** `xs->collect { in x : C; x.mass }` and `xs.{ in x : C; x.mass }` are typed by the body's result (`MassValue`), a nested collect by its innermost body, a body answering a sequence by every element type and `xs->collect f` by the named function's result; `select`, `reject` and `selectOne` keep the elements of `xs`, `reduce` follows its reducer's result — and the element a one-element collection hands back unreduced, unless the collection is known to hold two or more, by its own multiplicity, one it inherits by redefinition, or a chain through such features; a collection holding one at most is never reduced, so its element alone is the result — and `forAll`/`exists` stay `Boolean`, each with the multiplicity the Kernel Function Library declares. A body whose result cannot be typed keeps the library's `Anything`. Value conformance, a feature's bound value, invocation arguments, trigger arguments and enumerated values are judged by the specialized type, so `accept when counts.{in n : Integer; n}` is refused where it was silent, and `when counts.{in n; n > 3}` is accepted where it was refused. A scalar literal a body writes out is as exact as one bound directly: `attribute i : Integer = xs.{ in x : C; 1.5 }` is refused, and a quantity it writes out is measured against the target's dimension: `attribute t : DurationValue = xs.{ in x : C; 5 [m] }` is refused. `xs.?{…}` binds and is typed as `xs->select {…}` is — `(v1, v2).?{ in v : Vehicle; true }` is a `Vehicle` collection, not the `Anything` the sequence is; elements of sibling types share their nearest common supertype, so `(truck, car).?{ in v : Vehicle; true }` is a `Vehicle` collection too — and an element that is itself a collection value binds by the elements it holds, so `attribute i : Integer = xs.{ in x : C; xs.{ in y : C; 1.5 } }` is refused. A collection over `()` or a feature admitting no value keeps its declared type — `()->collect { in a : Integer; "s" }` is a `String` collection — but holds no element, so none is judged and its `reduce` takes nothing from an element it would hand back unreduced; so does one mapping every element to a `[0]` feature or function result — the multiplicity read through an alias, or from the feature or result a redefinition inherits it from — or any operation over such a collection, and an argument holding nothing is judged against no parameter type. An argument or constructor value that is a collection binds each element it holds on its own, so `Sail(vs.{ in v : Vehicle; (v, boat) })` and `new Fleet(vs.{ in v : Vehicle; 1.5 })` are refused by the element that does not bind where they were silent. A collection value binds as many values as it is known to hold, counted against the feature's multiplicity — `part b : Boat[1] = pair.items.{ in v : Vehicle; boat }` binds two — a `reduce` counting the one element it hands back unreduced or what its reducer yields over two or more, so `pair.items->reduce { in a : Vehicle; in b : Vehicle; (a, b) }` binds two and one over `()` none. A body reading the feature it values terminates as a self-referential argument does.
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