Is there a way of checking whether a value position is undefined?
Specifically, I'm trying to recognize the undefined value put in
structures by SHARED. It appears that a undefined value is sometimes
EQ? to another one, but this doesn't seem to be specified in the
documentation (ie, that there is only one undefined value).
Thanks!
==========
Separately, I tried to get inspiration from Typed Racket, but I'm not
sure it works well with SHARED even though it's in the TR language.
To wit:
-----
#lang typed/racket
(define-struct: m ([x : Number]))
(shared ([-a- (make-m -b-)]
[-b- 3])
-a-)
-----
produces an error at the binding of -a- saying
Type Checker: insufficient type information to typecheck. please add
more type annotations in: -a-
(NB: The error message could use some sprucing up.)
So I instead tried
(shared ([#{-a- : m} (make-m -b-)]
[#{-b- : Number} 3])
-a-)
and got several errors:
Type Checker: untyped identifier make-reader-graph imported from
module <scheme/base> in: (shared ((-a- (make-m -b-)) (-b- 3)) -a-)
. Type Checker: untyped identifier make-reader-graph imported from
module <scheme/base> in: (shared ((-a- (make-m -b-)) (-b- 3)) -a-)
. Type Checker: Bad arguments to polymorphic function in apply:
Domain: a ... a
Arguments: Nothing *
in: (shared ((-a- (make-m -b-)) (-b- 3)) -a-)
. Type Checker: Expression should produce 2 values, but produces 1
values of types Nothing in: (shared ((-a- (make-m -b-)) (-b- 3)) -a-)
Shriram
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