On 10/13/2010 10:41 AM, Jens Axel Søgaard wrote:
Hi All,
I am experimenting with syntax-parse in order to generate precise
error messages with a minimum of fuss. So far it looks very promising,
but I have a few questions.
Below I have systematically define syntax-classes for each
non-terminal in the grammar of cas-cade-e expressions.
I have used this grammar:
;;; SYNTAX
;;<cas-cad-e> ::= (cas-cad-e<val-expr> <case-clause> ...+)
;;<case-clause> ::= [(<datum> ...)<then-expr> ...+]
Each syntax class is given a description, which is included in
an error message, when some expression fails to parse (and
it is expected to).
To focus on the error messages I have removed the expansion.
Below I have tried to test which error messages I got.
For (casc) I get the correct error message, namely
casc: expected<cas-cad-expr> of the form (cas-cad-e<value-expr>
<case-clause> ...+) after 1 term in: (casc)
For (casc #:foo) I expected the error:
"casc: expected value-expr in: #:foo"
but got "casc: expected expr in: #:foo".
How do I make syntax-parse give the precise error?
syntax-parse considers "expr" the more precise error in this case, since
it occurs within the other syntax class (value-expr). I've considered
adding a "role" annotation to pattern variables, orthogonal to syntax
classes, that would let you express something like "expected expression
for the clause value". But I haven't added it yet.
In the meantime, you can make value-expr opaque (#:opaque), and that
should mask out the expr error.
For (casc 3 [(4) ]) where the<then-expr> ...+ part is omitted
I get the error message:
; casc: repetition constraint violated in: (casc 3 ((4)))
It is of course correct that the "at least one" repitition contraint
is violated,
but how do I get syntax-parse to tell me, that it is due to a missing
<then-expr> in a<case-clause>?
The generic error message "repetition constraint violated" has been
removed in recent versions of syntax-parse, BTW.
At the moment, the only way to customize the error message here is to use
((~between te:then-expr 1 +inf.0 #:name "result expression") ...)
or see the other options of ~between.
Ryan
#lang racket
(require
racket/stxparam
(for-syntax unstable/syntax ; for generate-temporary
syntax/parse
syntax/stx))
;;; SYNTAX
;;<cas-cad-e> ::= (cas-cad-e<val-expr> <case-clause> ...+)
;;<case-clause> ::= [(<datum> ...)<then-expr> ...+]
(define-syntax-parameter break
(λ (stx) (raise-syntax-error 'break "The keywork break was used of
context." stx)))
(define-syntax (casc stx)
(define-syntax-class datum
#:attributes (d)
#:description "<datum> part of the literal list in a<case-clause>"
(pattern d:expr))
(define-syntax-class then-expr
#:attributes (e)
#:description "an expression part to be part of a<case-clause>"
(pattern e:expr))
(define-syntax-class case-clause
#:attributes ((d 1) (te 1))
#:description "<case-clause> of the form [(<datum> ...)<then-expr> ...+"
(pattern [(d:datum ...) te:then-expr ...+]))
(define-syntax-class value-expr
#:attributes (e)
#:description "value-expr"
(pattern e:expr))
(define-syntax-class cas-cad-expr
#:attributes (ve (c 1) last-c)
#:description "<cas-cad-expr> of the form (cas-cad-e<value-expr>
<case-clause> ...+)"
(pattern (_ ve:value-expr c:case-clause ... last-c:case-clause)))
(syntax-parse stx
[usage:cas-cad-expr
#'42]))
;; Expressions that must signal an error
(casc) ; correct error message
;(casc #:foo) ; I expected the error: "casc: expected value-expr
..." but got "expected expr ..."
;(casc 3 [(4) ]) ; casc: repetition constraint violated in: (casc 3 ((4)))
;(casc 3 [(4) 2 3] [3]) ; only 3 is red shouldn't that be [3] ?
;; And a legal expression
(casc 3
[(1) 'one]
[(3) 'three])
--
Jens Axel Søgaard
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