alex-eg
1f445f0f9b
Two things out of one mechanism. `_' as a type means "work it out from
the initializer", so (var n _ (strlen s)) stops needing size-t spelled
out; `type-of' hands a macro the type of an expression, so a macro can
dispatch on what it was handed rather than on what was declared. Both
read the same answers from two sides.
Algorithm W's core, intra-procedural, with the extensions C forces:
- an unknown type, since (include stdio.h) brings in names we never
parsed. Unification is consistency rather than equality, so
anything touching an unparsed declaration stops constraining
instead of rejecting a program that compiled yesterday;
- the usual arithmetic conversions, since `+' is not a function of
one type;
- checking mode for initializers, since #(0 0) has no type of its own
and takes one from its context. #(T : ...) is the way out of that.
What it wanted on the way:
- what type a *name* has, which neither the typedef nor the tag
database recorded. One table serves functions and variables, since
a function type already has a surface spelling;
- a scope chain, so a (var c int 9) inside a do ends with the block;
- form-type, keyed by cons cell, so one form has one type;
- macros expanded during the walk rather than before it, so type-of
is answered in the scope the macro was written in.
Closures take the same machinery: a receiver whose type comes from a
call, captures written (name expr) and typed from the expression, and
conversion from a bare function wherever a closure is expected.
type-match grew `_' on the pattern side, since (closure ((int)) int)
and (closure ((float)) int) were separate clauses for one case.
240 lines
8.8 KiB
Scheme
240 lines
8.8 KiB
Scheme
;;; The type database.
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;;;
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;;; Every named aggregate, typedef and define the semantic engine
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;;; walks past is recorded here, so that macros (or other forms) can
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;;; ask what a type is made of. That is what lets a macro generate
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;;; code from a struct's fields given nothing but its name.
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;;;
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;;; Entries are filled in as toplevel forms are processed, in order, so
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;;; a type has to be declared before the macro that asks about it.
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(import
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scheme
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(scheme base)
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(chicken base)
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srfi-1
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srfi-69)
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;;; Two namespaces: `struct point' and a `point' typedef are separate
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;;; declarations. Tags -- struct, union and enum alike -- share the
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;;; second table between them
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(define +type-db+ (make-hash-table)) ; typedefs and defines
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(define +tag-db+ (make-hash-table)) ; struct, union and enums
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;;; What type a *name* has, which neither of the two above records:
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;;;
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;;; (fn sum ((a int) (b int)) int ...) -> (fn ((int) (int)) int)
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;;; (var origin (struct point) ...) -> (struct point)
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(define +name-db+ (make-hash-table))
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(define (strip-pub form)
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(if (eq? (car form) 'pub) (cdr form) form))
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(define (comment-form? f)
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(and (pair? f) (eq? (car f) 'comment)))
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;;; Fields are written with the type last and one or more names before
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;;; it, so ((x y int) (p (* char))) describes three fields. Flatten that
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;;; into one (name type) per field, which is what a caller wants.
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(define (normalize-fields fields)
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(append-map
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(lambda (field)
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(if (comment-form? field)
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(list)
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(let ((type (last field))
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(names (drop-right field 1)))
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(map (lambda (name) (list name type)) names))))
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(remove comment-form? fields)))
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;;; ([pub] struct name (fields ...) . attrs)
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(define (aggregate-fields form)
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(let ((f (strip-pub form)))
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(if (and (pair? (cddr f)) (list? (caddr f)))
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(caddr f)
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(list))))
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(define (add-struct name form)
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(hash-table-set! +tag-db+ name
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(list 'struct name (normalize-fields (aggregate-fields form)))))
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(define (add-union name form)
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(hash-table-set! +tag-db+ name
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(list 'union name (normalize-fields (aggregate-fields form)))))
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;;; ([pub] enum name (value ...))
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(define (add-enum name form)
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(let ((f (strip-pub form)))
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(hash-table-set! +tag-db+ name
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(list 'enum name (if (and (pair? (cddr f)) (list? (caddr f)))
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(caddr f)
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(list))))))
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;;; ([pub] typedef new-name target)
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(define (add-typedef name form)
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(hash-table-set! +type-db+ name
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(list 'typedef name (last (strip-pub form)))))
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;;; (define name value ...) -- a C #define, kept so a macro can read a
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;;; compile-time constant rather than re-parse the source.
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(define (add-define name form)
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(hash-table-set! +type-db+ name
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(list 'define name (cddr (strip-pub form)))))
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;;; `(type-of x)' inside a macro body: the type of the expression the
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;;; macro was handed, where `get-name-type' only answers for a name.
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;;; The walker that can answer it lives in `semen', which is compiled
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;;; after this, so it installs itself here for the length of one
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;;; expansion. Outside one there is no scope to ask about, and the
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;;; answer is #f.
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(define current-type-of (make-parameter (lambda (form) #f)))
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(define (type-of form) ((current-type-of) form))
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(define (add-name-type! name type)
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(hash-table-set! +name-db+ name type))
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;;; #f for a name never declared, which is what `printf' looks like
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;;; until something parses stdio.h. Not an error here; the caller
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;;; decides.
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(define (get-name-type name)
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(hash-table-ref/default +name-db+ name #f))
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;;; What `(make-adder 10)' has for a type: `make-adder's return type,
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;;; or #f when NAME is not a function with a signature on record
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(define (get-return-type name)
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(let ((type (get-name-type name)))
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(and (pair? type)
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(eq? 'fn (car type))
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(= 3 (length type))
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(third type))))
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(define (get-tag-info name)
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(hash-table-ref/default +tag-db+ name #f))
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;;; First look up the ordinary identifier, then tag of that id when
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;;; no ordinary one was declared, as C does
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(define (get-type-info name)
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(or (hash-table-ref/default +type-db+ name #f)
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(get-tag-info name)))
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;;; ((name type) ...) for a struct or union, #f for anything else --
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;;; including a name that was never declared. Callers give the better
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;;; error, since they know what they wanted it for.
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;;;
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;;; A typedef is followed to what it stands for, so reflection over an
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;;; alias works exactly as it does over the name it aliases.
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(define (get-fields name)
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(let ((info (resolve-type-info name)))
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(and info
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(memq (car info) '(struct union))
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(caddr info))))
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;;; Follow a typedef chain to the name it stands for. #f if NAME is
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;;; not a typedef. A typedef that leads back to itself stops rather
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;;; than spinning: nothing prevents one from being written.
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(define (get-underlying-type name)
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(let follow ((name name) (seen (list)))
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(and (not (member name seen))
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(let ((info (get-type-info name)))
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(and info
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(eq? (car info) 'typedef)
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(let ((target (caddr info)))
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(or (and (symbol? target) (follow target (cons name seen)))
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target)))))))
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;;; The declaration NAME ultimately names. For a typedef that is the
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;;; entry of whatever it stands for, and for anything else it is
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;;; NAME's own info
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(define (target-tag target)
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(cond
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((symbol? target) target)
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((and (pair? target)
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(memq (car target) '(struct union enum))
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(pair? (cdr target))
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(symbol? (cadr target)))
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(cadr target))
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(else #f)))
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(define (resolve-type-info name)
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(let ((info (resolve-ordinary-type-info name)))
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(if (and info (memq (car info) '(struct union enum)))
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info
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(or (get-tag-info name) info))))
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(define (resolve-ordinary-type-info name)
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(let ((info (get-type-info name)))
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(and info
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(if (eq? (car info) 'typedef)
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(let ((tag (target-tag (get-underlying-type name))))
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;; A typedef target is written in type position, where
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;; `struct point' means the tag
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(and tag (or (get-tag-info tag) (get-type-info tag))))
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info))))
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;;; Type matcher macro
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;;; (type-match type
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;;; (int ...)
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;;; ((* const char) ...)
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;;; ([int 10] ...)
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;;; ((* _) ...) ; a pointer to anything
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;;; ((¤ _ _) ...) ; an array of anything, any length
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;;; (else ...))
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;;;
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;;; A type is a form, not an atom, so patterns are matched structurally
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;;; rather than dispatched on like `case'. They are literal types and
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;;; are not evaluated; `else' is optional and the whole thing is #f when
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;;; nothing matches and there is no else.
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;;;
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;;; `_' in a pattern matches anything in that position, the same thing
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;;; it means in a type. Without it every spelling has to be enumerated:
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;;; `(closure ((int)) int)' and `(closure ((float)) int)' are separate
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;;; clauses for what is one case.
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;;;
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;;; A `_' written last takes everything that remains, because a type's
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;;; words are spread rather than nested -- `(* const char)' is three
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;;; elements, so `(* _)' has to cover two of them to mean "a pointer to
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;;; anything".
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;;;
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;;; Nothing destructures: a macro body is ordinary Scheme and a type is
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;;; a list, so `(caddr (type-of x))' already reads the length out of
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;;; `(¤ int 4)'.
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(define-syntax type-match
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(syntax-rules (else)
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((_ type) #f)
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((_ type (else body ...)) (begin body ...))
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((_ type (pattern body ...) clause ...)
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(if (type-pattern-matches? 'pattern type)
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(begin body ...)
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(type-match type clause ...)))))
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(define (type-pattern-matches? pattern type)
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(cond
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((eq? pattern '_) #t)
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((and (pair? pattern) (pair? type))
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(if (and (eq? (car pattern) '_) (null? (cdr pattern)))
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#t ; a trailing `_' takes the rest
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(and (type-pattern-matches? (car pattern) (car type))
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(type-pattern-matches? (cdr pattern) (cdr type)))))
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(else (equal? pattern type))))
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;;; Map function to each field/value of a structure/union/enum
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;;; For enums, field-type is the type of the enum (since C 23)
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;;; (map-fields type-name
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;;; (lambda (field-name field-type) ...))
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;;;
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;;; Returns #f if nothing of that name was declared
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(define (map-fields struct-union-enum fn)
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(let ((info (resolve-type-info struct-union-enum)))
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(and info
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(case (car info)
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((struct union)
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(map (lambda (field) (fn (car field) (cadr field)))
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(caddr info)))
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;; An enumerator's type is the enum itself -- named as it was
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;; declared, since `enum some-typedef' is not a C type.
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((enum)
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(let ((type (list 'enum (cadr info))))
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(map (lambda (value) (fn value type))
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(caddr info))))
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(else #f)))))
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