Comments (5)
The runtime behavior of this example is as expected. I agree that the actual error you get is terrible, however.
I have considered making it a syntax error to write something like Γ
in a "term" position unless there was a binding for it, so you could get a syntax error. The trouble with that is that people often like to use x
for both a non-terminal and for a literal.
The trouble with the other interpretation of the nonterminals (making them binding in contracts), is that a contract like this one:
(define-metafunction stlc
same-constructor : e e -> #t or #f
[(same-constructor (λ (x_1 : t_1) e_1)
(λ (x_2 : t_2) e_2))
#t]
[(same-constructor (e_1 e_2) (e_3 e_4))
#t]
[(same-constructor x_1 x_2)
#t]
[(same-constructor e_1 e_2) #f])
would be not at all what you want (it would be like the current redex writing e_1
twice in the contract).
Do you have any ideas on how to improve the situation?
from redex.
Ah, that's a good point. In fact, I realize now that I use that feature in several of my contracts.
Making unbound non-terminals in term position an error seems almost like a good idea; perhaps making this the default and adding an optional argument to allow specified non-terminals to also be literals would be a good compromise. Given that x
seems like the most common instance of this, any non-terminal defined as variable-not-otherwise-mentioned
could default to being treated this way.
Otherwise, perhaps documenting this more clearly in define-metafunction
would be good enough. My first throught was to look at the documentation and see if I missed some hint about the behavior of #:pre
, but there aren't even examples of using it in the documentation.
from redex.
from redex.
Maybe so. I suppose we could make the current behavior the default, and opt-in to the additional checking? But if it's ad-hoc-ness you worry about, maybe not.
My first ad hoc thought was to bind non-terminals from the domain of the contract when a #:pre is given, and bind them in the co-domain when #:post is given, and document this behavior carefully. This would result in contracts and pre/post conditions being more readable, I think. But.. it might be subtley confusing behavior.
As for pull requests, I'll try get around to that today. There have been a handleful of typos I've been meaning to fix in the docs anyway.
from redex.
Sounds like confusing behavior to me.
I think there's nothing to be done here with this PR anymore, so I'm going to close it. If you want to re-open, please do so.
from redex.
Related Issues (20)
- Defining mutually recursvie judgments/meta-functions HOT 2
- Feature request: tag-with-names for apply-reduction-relation* HOT 2
- test-->>∃ #:steps n passes test if n > than the number of actual reduction steps for a term. HOT 4
- `traces` doesn't draw arrows after "fix layout" HOT 5
- `stepper` doesn't display whole term... unless I *narrow* its box?
- Feature Request: `stepper` with judgment-form use rule names
- Feature request: adjust font for `stepper` HOT 3
- Redex rejects literals at different ellipsis depths in binding form declarations HOT 1
- Generating random expressions sometimes results in an infinite loop
- where/error failures give no information about source of error HOT 6
- where/error failures don't tell you what failed to match
- using judgment-holds within define-relation HOT 1
- Redex's reduction graph windows sneakily hide part of term reductions HOT 1
- where/error doesn't report errors sometimes HOT 8
- define-metafunction does not check type signature on LHS HOT 3
- Docs missing a word I can't fill in. HOT 1
- `derivation/ps` always causes an error by calling a non-existent method HOT 1
- Default rendering font changed between Racket v8.8 and v8.9? HOT 4
- Redex PDF output contains a wrong font, while Racket REPL renders the correct font HOT 2
- Reduce space between rendered rules of a judgement form HOT 3
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from redex.