[Feature Request] Add unsafe-cast #1445
Description
Is your feature request related to a problem? Please describe.
Typed Racket uses predicates to refine types, but this approach has limitations since many types cannot have corresponding predicates. This creates situations where we can logically deduce more precise types for variables via tagged union, but have no way to express them in the type system.
Example:
(let ()
(define-type T (→ Any Void))
(let loop ([p : (∪ (→ T) T) (λ () (λ (v) (displayln v)))]
[tag : Natural 0])
(case/eq tag
[(0) (loop (p) 1)] ; p is (→ T) here
[(1) (p 123)]))) ; p is known to be T hereDescribe the solution you'd like
I propose adding operators that allow programmers to explicitly assert that a variable is or isn't of a certain type. This would solve the cases where predicates are insufficient.
Example:
#lang typed/racket/base
(module untyped-utils racket/base
(provide (rename-out [const unsafe-assert]
[const unsafe-assert-not]))
(define (const . _) #f))
(require typed/racket/unsafe)
(unsafe-require/typed 'untyped-utils
[unsafe-assert (∀ (a) (pred a))]
[unsafe-assert-not (∀ (a) (→ Any Boolean : #:+ (! a) #:- a))])
(require (for-syntax racket/base syntax/parse))
(define-syntax unsafe-asserts
(let ()
(define-syntax-class unsafe-asserts-clause
[pattern [x:id ((~datum !) t)]
#:with cond-clause
(syntax/loc #'x
[((inst unsafe-assert t) x)
(error "Assertion failed")])]
[pattern [x:id t]
#:with cond-clause
(syntax/loc #'x
[((inst unsafe-assert-not t) x)
(error "Assertion failed")])])
(λ (stx)
(syntax-parse stx
[(_ (c:unsafe-asserts-clause ...) body ...+)
(quasisyntax/loc stx
(cond c.cond-clause
...
[else body ...]))]))))
(require racket/case)
(let ()
(define-type T (→ Any Void))
(let loop ([p : (∪ (→ T) T) (λ () (λ (v) (displayln v)))]
[tag : Natural 0])
(case/eq tag
[(0) (unsafe-asserts ([p (! T)]) (loop (p) 1))]
[(1) (unsafe-asserts ([p T]) (p 123))])))Describe alternatives you've considered
Creating a separate package if this doesn't align with Typed Racket's design goals.
Do you want to contribute to this feature?
Yes, I am willing to implement this feature and submit a PR to provide unsafe-asserts in typed/racket/unsafe/assert.