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slight language tweak

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Aaron Weiss 2022-08-04 15:57:07 -07:00
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rfcs/exact-table-types.md
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@ -38,11 +38,11 @@ getx(t : {x: number}) return x end`, we treat is as _sealed_ and _inexact_ meani
parameter will _not_ change, but call sites to the function will permit tables that have _more_ parameter will _not_ change, but call sites to the function will permit tables that have _more_
properties than the ones listed. Within the body of the function however, the typechecker should properties than the ones listed. Within the body of the function however, the typechecker should
only permit the use of properties explicitly listed in the type since it was annotated. If the type only permit the use of properties explicitly listed in the type since it was annotated. If the type
was instead inferred, a dual condition should be true --- the properties explicitly listed in was instead inferred, a dual condition should be true --- the properties explicitly listed in the
the type are precisely the ones whose existence the function depends on. Meanwhile, when a table type are precisely the ones whose existence the function depends on. Meanwhile, when a table type is
type is used for a local definition, it is given an _unsealed_ and _exact_ type meaning we always used for a local definition, it is given an _unsealed_ and _exact_ type meaning we always know
know _precisely_ the set of properties it has, but that the type itself is stateful and assignments _precisely_ the set of properties it has, but that the type itself is stateful and assignments are
are allowed to grow the set of properties the table has. allowed to grow the set of properties the table has.
This alludes to a gap of two sorts of table types: __sealed, exact tables_ and _unsealed, inexact This alludes to a gap of two sorts of table types: __sealed, exact tables_ and _unsealed, inexact
tables_. We believe the latter is undesirable in the sense that there does not appear to be a tables_. We believe the latter is undesirable in the sense that there does not appear to be a
@ -100,9 +100,9 @@ their difference is not something we can distinguish contextually in general (as
_sealed_ and _unsealed_), we have to propose a syntax for them. This proposal posits that the best _sealed_ and _unsealed_), we have to propose a syntax for them. This proposal posits that the best
path forward is to make sealed, exact tables the default table that you get when you write, e.g. `{ path forward is to make sealed, exact tables the default table that you get when you write, e.g. `{
x: number, y: number }` as an annotation on a parameter or in a type alias. To annotate a table as x: number, y: number }` as an annotation on a parameter or in a type alias. To annotate a table as
inexact and therefore open to extension, we would include an ellipsis in the type, e.g. `{ x: inexact and therefore open to having additional properties, we would include an ellipsis in the
number, y: number, ...}`. This was originally proposed as an alternative design in the [width type, e.g. `{ x: number, y: number, ...}`. This was originally proposed as an alternative design in
subtyping][width-subtyping] proposal. the [width subtyping][width-subtyping] proposal.
There are a few apparent benefits to this approach. First, it means that a programmer designing an There are a few apparent benefits to this approach. First, it means that a programmer designing an
API has to make a conscious decision that "yes, in fact, it is sensible to treat all of the unlisted API has to make a conscious decision that "yes, in fact, it is sensible to treat all of the unlisted