Predicate types in BSL and BSL+

[bugcounting]

In the student languages, function predicate allows one to define a
type (signature form) constrained by a predicate that indicates
precisely the values that belong to the type.

(define (nonzero-number? v) (and (number? v) (not (= v 0))))
(define NonZeroNumber (signature (predicate nonzero-number?)))

Even though predicate is defined in all student languages, it cannot
really be used in BSL or BSL+ where functions are not first class.

I'm unsure what's the best way of handling this. Removing predicate
from BSL is not a good idea, given that there are several examples of
data type definitions that are already used with BSL and BSL+ (e.g.,
interval data types in HtDP) but would require predicate to be
defined.

Would making functions first class already in BSL and BSL+ create any
issues?

[racket-discourse-github-bot]

This issue has been mentioned on Racket Discourse. There might be relevant details there:

https://racket.discourse.group/t/signatures-in-student-languages/4166/4

[rfindler]

It seems reasonable that predicate could accept an identifier that's bound to a function in BSL.

[mfelleisen]

1. BSL and BSL+ are explicitly designed to avoid functionals because experience over years has shown that

• novice programmers make mistakes (that's a social theorem)
• error messages confound them
• and the presence of functionals -- even if not used intentionally -- causes the implementation to issue incomprehensible error messages.

So no, we won't change BSL/+ this way.

2. If predicate? were a plain higher-order function, we could export it via

(provide-higher-order-primitive
  ;  (Any -> Boolean) -> SignatureElement
 predicate? (f) )

But, signature is actually implementing a DSL (with parser and code generator @michaelballantyne ) manually, and after staring at @mikesperber 's code for a while, I am lost ... :-)

[mfelleisen]

p.s. The correct solution is to define such signatures outside and provide them to novices via teach packs.

p.p.s. More on this later on Discourse.

[shhyou]

FWIW, I just want to add to this

• and the presence of functionals -- even if not used intentionally -- causes the implementation to issue incomprehensible error messages.
  A typical example:

(define x 3)
(cond
  [zero?(x) 0]
  [else (- x 1)])

spotting the syntax error zero?(x) instead of getting a run-time error about "3 is not a function" (where 3 is the value of x) is helpful.

[mikesperber]

I tend to agree with @mfelleisen, even I don't see how allowing the name of a function specifically in that place would be harmful, similar to how animate works in the image teachpack. @mfelleisen, would you object if I made this work?

Is it an option to just add the signatures mentioned in HtDP (i.e. intervals) as primitives?

[mfelleisen]

Go ahead. I'd look at run-animate but as I said, this is all a DSL created manually. Perhaps an embedding factoring plus the provide-higher-order could make it all work.