This PR primarily changes how we represent Base types and unions of
Base types. Basically, Base types now contain a bits field, which
contains an exact-nonnegative-integer? with exactly one bit set to
1. This allows us to represent unions of Base types by simply OR-ing
together the various base bits. We store these unions in a new type
called a BaseUnion (which contains a field for numeric Base type bits
and a field for non-numeric Base type bits). We can perform set
operations on BaseUnion types rather quickly (using simple bitwise
arithmetic operations).
To make Union and BaseUnion work together nicely, the Union type now
has a field for non-Base types, and a field which contains any and all
Base types placed directly in the Union ( either as a Base if there is
only one, or as a BaseUnion if there are more than one).
Other changes present in this PR:
Base types are now "closed" -- i.e. Base types are only declared in
base-types.rkt and numeric-base-types.rkt with a special macro that
assigns them their respective bits. The constructor make-Base is no
longer provided for miscellaneous usages.
Some singleton Value types were moved to Base so all of our common
unions of basic types can fit into the new BaseUnion type (namely
Null, Void, One, Zero, and the booleans).
A new Val-able match expander lets us match on singleton types that
used to all be Value types, but, as described above, now some are Base
types.
Unions contain deterministically ordered, duplicate free lists (in
addition to sets for equality and constant time membership checks), so
iterating over Unions can be done deterministically (yay!) -- this
gets rid of some otherwise problematic behavior in areas like type
inference, where the order Unions are iterated over can actually
affect the results (i.e. if two valid type inferences are possible,
nondeterministic ordering means we can sometimes get one and sometimes
get another, which makes for particularly difficult to debug issues
and in general has no immediate solution (both substitutions are
valid, after all!))
Prior to this change (which was Typed Racket PR 469) all internal TR
objects (Reps) were interned and kept around for the entire duration
of type checking. Because of this, frequent operations that rebuilt
types were particularly costly (e.g. various forms of
substitution). To recoup some of this cost, caching was being used in
a lot of places. This PR sought to remove interning as the default
behavior for Reps and allow for more flexibility in how we approach
time/space performance needs going forward.
The following changes were included in this overhaul:
Interning: All Reps are no longer interned. Right now we only intern
unions and some propositions.
Rep generic operations: we now use racket/generic so we're not
reinventing this wheel.
Singletons: Reps (e.g. TrueProp, Univ, etc) can be declared singleton,
which creates a single instance of the rep that all visible operations
(even within the declaring module) are defined in terms of
(e.g. predicates are defined as (λ (x) (eq? x singleton-instance)),
etc).
Custom constructors: Custom constructors can be specified for Reps,
which allows for simple normalization, interning, or other invariants
to be enfored whenever a Rep is created.
Union: Unions used to try to ensure no obviously overlaping types
would inhabit the same Union (e.g. (U String (Pairof Any Any) (Pairof
Int Int)) would be simplified to (U String (Pairof Any Any))). This,
however, required frequent calls to subtyping every time a Union was
modified and working with Unions thus had a high cost (another thing
that caching was used to reduce). Instead of this, Unions now enforce
a much simpler set of invariants on their members: (1) No duplicates
(by virtue of using a hash-based set), (2) Any and Nothing do not
appear in unions, and (3) Nested unions are flattened. Also, using a
hashset as the internal data structure meant that we could easily
intern unions w.r.t. equal? equality. NOTE: we do reduce unions to not
contain obviously overlapping terms when printing to users and when
generating contracts (so obviously and avoidable inneficient contracts
are not generated – See union.rkt for 'normalize-type').
Subtyping changes: Subtyping has been designed to reduce dispatch time
w/ a switch since we no longer cache _all_ subtyping calls (we only
cache subtyping results for unions since they have some costly
subtyping).
prop-ops changes: AndProps now are careful to sort OrProps by length
before building the resulting proposition. This is done because
OrProp implication only checks if one Or is a subset of another Or.
By ordering Or props by size, we only ever check if an OrProp implies
another if its size is <= the other OrProp. This also makes the
smart constructor '-and' more robust, since the order the props
appear does not affect if some Ors are kept or not.
Testing: More subtype tests have been added (we are still probably
relying too much on typecheck-tests.rkt and not the more granular unit
tests in general). Also, typecheck-tests.rkt has been changed to
check for type-equivalence (i.e. subtyping in both directions)
instead of equal? equivalence.
This is a major to some of the internal representation of things
within Typed Racket (mostly affecting structs that inherited from Rep
(see rep/rep-utils.rkt)), and lots of tweaks and bug fixes that
happened along the way.
This PR includes the following major changes:
A new rep-utils implementation, which uses struct properties for the
generic operations and properties of the various Reps (see
rep-utils.rkt)
More specific Rep inheritance (i.e. arr no longer inherits from Type,
because it is not a Type, etc ...) (see type-rep.rkt, core-rep.rkt,
values-rep.rkt), and thus things like Type/c no longer exist
New Rep's to classify the things that are no longer Type or Prop,
(such as PropSets, SomeValues, Results, etc -- see core-rep.rkt and
values-rep.rkt)
uses of type-case now replaced by uses of Rep-fold and Rep-walk
structural types can specify their fields' variance and operations
like subtyping and free-vars can generically operate over these types
(see type-rep.rkt)
type-mask replaces types key -- types masks are described in detail in
(rep/type-mask.rkt)
Types can specify a predicate to recognize their "top type" via [#:top
pred])
There is an explicit 'Bottom' type now (i.e. neither union or
intersection are used)
subtyping re-organized, slight tweaking to inference
various environments got for-each functions in addition to the map
functions they had (e.g. type-name-env.rkt)
Empty is no longer an Object? -- the OptObject? predicate checks for
either Object or Empty, and so it is easier to be clear about where
Empty makes sense appearing and where it does not
Previously signatures were created with promises in their fields, now
we create a promise around each signature (this way the contracts for
Signature fields are cleaner)
Names for structs now use the args field to describe how many type
arguments they take (Note: this could use further tidying for sure!)
simplified the propositional logic code in several places, got rid of
escape continuations, etc (see prop-ops.rkt, tc-envops.rkt,
tc-metafunctions.rkt)
we now use subsumption more to simplify type results from type
checking, e.g. if the type does not overlap w/ false, it's false
proposition is FalseProp, etc (see tc-expr-unit.rkt and prop-ops.rkt,
the function is called reduce-tc-results/subsumption)
updating along a path will now intersect with the expected structural
type if it is not encountered (e.g. updating Any with (Int @ car) now
produces (Pairof Int Any) instead of Any -- see update.rkt)
lots of tests were tweaked to match up w/ the new prop subsumption
that occurs
remove was renamed subtract (so as to not conflict w/ racket/base's
remove)
a restrict function was added, which acts like intersect but is never
additive (i.e. it will never create an intersection if it can't figure
out how the two types relate -- see intersect.rkt)
tc-subst was modified to substitute out all the variables leaving
scope at once (and I simplified/tweaked some of the logic in there a
little, see tc-subst.rkt)
Type checking function applications now propagates information learned
why type checking the arguments, (e.g. (begin (f (assert x boolean?))
...)) ; the remainder of the begin is aware that x is a boolean)
Adds intersection types as a better way to handle the the case
when restrict cannot structurally intersect two types (e.g. when
you learn within a polymorphic function a variable x of type A
is also an Integer, but we dont know how A relates to Integer).
This allows for non-lossy refinements of type info while typechecking.
This pull request is largely a renaming effort to clean up the TR codebase. There are two primary things I wanted to change:
1. Replace all occurrences of "filter" with "prop" or "proposition"
- The word "filter" is a meaningless opaque term at this point in the Typed Racket implementation. If anything, it just adds confusion to why things are the way the are. We should use "proposition" instead, since that's what they actually are.
2. Avoid using "Top" and "Bottom" in both the type and proposition realm.
- Currently the top type is called Univ and the bottom type is called bottom, while the top proposition is called Top and the bottom proposition is called Bot. This is just unnecessarily confusing, doesn't really line up w/ the user-space names, and doesn't line up with the names we use in TR formalisms. Worse, all of the top types of primitive types---e.g. the type of all structs, StructTop--- use Top, so it is really easy to get confused about what name to use for these sorts of things.
With these issues in mind, I made the following changes to names:
Top -> TrueProp
Bot -> FalseProp
TypeFilter -> TypeProp
NotTypeFilter -> NotTypeProp
AndFilter -> AndProp
OrFilter -> OrProp
-filter t o -> -is-type o t
-not-filter t o -> -not-type o t
FilterSet -> PropSet
NoFilter -> #f
NoObject -> #f
-FS -> -PS
-top -> -tt
-bot -> -ff
implied-atomic? q p -> implies-atomic? p q
filter-rec-id -> prop-rec-id
-no-filter -> -no-propset
-top-filter -> -tt-propset
-bot-filter -> -ff-propset
-true-filter -> -true-propset
-false-filter -> -false-propset
PredicateFilter: -> PredicateProp:
add-unconditional-filter-all-args add-unconditional-prop-all-args
This makes a type like
(-> Any Boolean : #:+ Integer)
a subtype of a type like
(-> Any Boolean : #:+ Number)
For not filters, the direction is reversed.
