[racket] Typed Racket frustration

For something like that, this works, and supports polymorphism, checks an even number of elements, and separates the key types from the value types:
#lang typed/racket
(: mb-hash : (All (A B) (Rec lst (U Null (List* A B lst))) -> (HashTable A B)))
(define (mb-hash xs)
  (let loop ([hsh : (HashTable A B) (hash)] [xs : (Rec lst (U Null (List* A B lst))) xs])
    (match xs
      [(list) hsh]
      [(list-rest a b rst) (loop (hash-set hsh a b) rst)])))
(define xs '(width 10 foo bar zam 42))
(mb-hash xs)
;'#hash((width . 10) (foo . bar) (zam . 42))

Before the repo was split, there was a pull request by Asumu Takikawa here:
https://github.com/plt/racket/pull/564
which would allow something like this to be used for a type for hash, where the arguments wouldn’t have to be in a list like this.

On Jan 24, 2015, at 2:50 PM, Matthew Butterick <mb at mbtype.com> wrote:

> For this issue, Alex just submitted a patch, so it will be fixed soon.
> 
> In general, TR doesn't work well with functions like `hash` that take
> their arguments in pairs or triples, and thus they end up having
> simpler types than the full Racket behavior.
> 
> 
> In my case, I've already committed to indefinite-arity `hash` as part of the program interface (a decision I could reconsider, though it would require a new quantum of benefit to be worthwhile)
> 
> Am I wrong that this TR function approximates the standard `hash` (by unpacking the arguments manually)? 
> 
> If not, is this approach disfavored for some other reason?
> 
> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
> #lang typed/racket/base
> 
> (: mb-hash ((Listof Any) . -> . HashTableTop))
> (define (mb-hash xs)
>   (let-values ([(ks vs even?) (for/fold 
>                          ([ks : (Listof Any) null][vs : (Listof Any) null][even? : Boolean #t])
>                          ([x (in-list xs)])
>                           (if even?
>                               (values (cons x ks) vs #f)
>                               (values ks (cons x vs) #t)))]) 
>     (when (not even?) (error 'bad-input))
>     (for/hash ([k (in-list ks)][v (in-list vs)])
>       (values k v))))
> 
> (define xs '(width 10 foo bar zam 42))
> (mb-hash xs) 
> ; '#hash((width . 10) (foo . bar) (zam . 42))
> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
> 
> 
> 
>  
> 
> 
> 
> On Sat, Jan 24, 2015 at 9:27 AM, Sam Tobin-Hochstadt <samth at cs.indiana.edu> wrote:
> On Sat, Jan 24, 2015 at 12:06 PM, Matthew Butterick <mb at mbtype.com> wrote:
> > Of course, developer time is limited, so I'm always reluctant to post
> > comments amounting to "it would be nice if someone else did such-and-such
> > for me ..." In this case I simply don't have the experience with TR to know
> > where the major breaks with idiomatic Racket occur.
> >
> > But user time is limited too. In my case, I'm trying to decide whether TR is
> > a cost-effective upgrade for my Racket program. What I can't figure out is
> > whether `hash` is an outlier, or the tip of the iceberg, because today, I'm
> > a complete idiot about TR.
> 
> For this issue, Alex just submitted a patch, so it will be fixed soon.
> 
> In general, TR doesn't work well with functions like `hash` that take
> their arguments in pairs or triples, and thus they end up having
> simpler types than the full Racket behavior.
> 
> Sam
> 
> >
> >
> >
> >
> > On Sat, Jan 24, 2015 at 9:01 AM, Matthew Butterick <mb at mbtype.com> wrote:
> >>
> >> Of course, developer time is limited, so I'm always reluctant to post
> >> comments amounting to "it would be nice if someone else did such-and-such
> >> for me ..." In this case I simply don't have the experience with TR to know
> >> where the major breaks with idiomatic Racket occur.
> >>
> >> But user time is limited too. In my case, I'm trying to decide whether TR
> >> is a cost-effective upgrade for my Racket program. What I can't figure out
> >> is whether `hash
> >>
> >>
> >> On Sat, Jan 24, 2015 at 7:13 AM, Robby Findler
> >> <robby at eecs.northwestern.edu> wrote:
> >>>
> >>> It seems like a small step to start by documenting the ones that
> >>> people trip up against multiple times in our mailing lists.
> >>>
> >>> Robby
> >>>
> >>> On Sat, Jan 24, 2015 at 9:07 AM, Matthias Felleisen
> >>> <matthias at ccs.neu.edu> wrote:
> >>> >
> >>> > On Jan 24, 2015, at 1:43 AM, Matthew Butterick wrote:
> >>> >
> >>> > FWIW, Sam's point that one can't expect every untyped program to work
> >>> > without modification is entirely fair.
> >>> >
> >>> >
> >>> > Correct.
> >>> >
> >>> > But Konrad's point is also fair: if a function like `append` or `hash`
> >>> > works
> >>> > differently in TR, then it is, for practical purposes, not the same
> >>> > function, even if it relies on the same code.
> >>> >
> >>> >
> >>> > This statement is too coarse. There are at least two senses in which a
> >>> > TR
> >>> > function f is distinct from an R function:
> >>> >
> >>> > 1. f's type restricts the usability of f to a strict "subset" [in the
> >>> > naive
> >>> > set-theoretic sense]. This is most likely due to a weakness of the type
> >>> > system; the language of "theorems" isn't strong enough to express R's
> >>> > intention w/o making the inference rules unsound. [Unlike in the legal
> >>> > world, In PL arguments of 'typedness' must be about truly-guilty or
> >>> > not-guilty. The rulings are completely impartial and uniform, i.e.,
> >>> > totally
> >>> > fair.]
> >>> >
> >>> > 2. f's type ___changes___ the meaning of the code. (That's also
> >>> > possible but
> >>> > I don't want to fan the argument that Sam and I have about this.)
> >>> >
> >>> >
> >>> > If it would be superfluous to repeat every TR function in the
> >>> > documentation,
> >>> > it still could be valuable to document some of the major departures
> >>> > from
> >>> > Racket. I mean, I would read that, for sure ;)
> >>> >
> >>> >
> >>> >
> >>> > Actually it would not be superfluous. We just don't have the manpower
> >>> > but
> >>> > perhaps it's time to tackle this problem (perhaps in a semi-automated
> >>> > manner).
> >>> >
> >>> > -- Matthias
> >>> >
> >>> >
> >>> > ____________________
> >>> >   Racket Users list:
> >>> >   http://lists.racket-lang.org/users
> >>> >
> >>
> >>
> >
> >
> > ____________________
> >   Racket Users list:
> >   http://lists.racket-lang.org/users
> >
> 
> ____________________
>  Racket Users list:
>  http://lists.racket-lang.org/users

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