> On Mar 19, 2018, at 11:37 AM, David Storrs <david.sto...@gmail.com> wrote:
>
> This does not work:
> (define user (hash 'name "bob" 'job (hash 'type 'dev 'id 112)))
> (match user
> [(hash-table ('name name) ('job (hash-table 'type type 'id id)))
> (displayln name)
> (displayln type)
> (displayln id)])
> ; id49: unbound identifier;
> ; also, no #%top syntax transformer is bound
> ; in: id49
> ; [,bt for context]
Jos Koot already found the problem, but why is this error message so horrible?
This seems like a bug in match.
According to the Docs, `hash-table` patterns look like one of these, where ooo
means a program-level ellipsis:
(hash-table (pat pat) ...)
(hash-table (pat pat) ...+ ooo)
The pattern (hash-table 'type type 'id id) doesn't follow one of those forms,
but it match seems to parse it anyway.
The reason is that there is a secret undocumented, unspecified form of the
`hash-table` pattern:
(hash-table p ...)
where p is either (pat pat) or id. If it's an id, then it matches a key-value
pair.
So according to this secret feature, the pattern means something completely
different that you never meant it to mean.
For extra weirdness:
#lang racket
(define user (hash 'type 'dev 'id 112))
(match user
[(hash-table 'type x 'id y)
(displayln x)])
;quote16: unbound identifier;
; also, no #%top syntax transformer is bound in: quote16
This secret feature of `hash-table` patterns is wreaking havoc on everything.
Alex Knauth
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