Re: [racket-users] Werid contract violation blame erros

[epi via Racket Users]

Thank you, Robby,

This clarifies it a little bit. So far I've avoided reading about the racket 
module system in details, but I think now is as good time as ever.
April 7, 2021 10:40 PM, "Robby Findler" mailto:ro...@cs.northwestern.edu?to=%22Robby%20Findler%22%20)>
 wrote:
The short answer: you probably should use (provide (contract-out)) instead 
of define/contract.

The slightly longer answer: when you write a contract, you are not just 
describing what the legal inputs and outputs are, you are also establishing a 
*boundary* between two regions of code. In the case of define/contract, you are 
establishing a boundary between the function (here: f) and the module that it 
contains (here the file "a.rkt"). In the case of (provide (contract-out ...)) 
the boundary is between a.rkt and b.rkt.

The slightly uncomfortable answer: it is my (not completely solid yet) opinion 
that the boundary that is drawn for define/contract is perhaps not the right / 
best one. In a theoretical/mathematical sense it is a completely fine and 
completely defensible one. But it trips people up sometimes. (There are 
examples others have posted in the past that are perhaps even stranger than 
yours but boil down to the same specific design choice for define/contract.)

Robby
On Wed, Apr 7, 2021 at 2:10 PM epi via Racket Users 
mailto:racket-users@googlegroups.com)> wrote: 
Hello Racket users,

I am trying to understand a contract violation message that I am getting.
Here is the file a.rkt:

#lang racket
(provide f)
(define/contract (f a)
(-> boolean? any/c)
'())

and this is b.rkt:

#lang racket
(require "a.rkt")
(f 3)
I would expect that the caller is blamed for the contract violation, but the 
error message that I get is as follows:
f: contract violation
expected: boolean?
given: 3
in: the 1st argument of
(-> boolean? any/c)
contract from: (function f)
blaming: /home/epi/snippets/a.rkt
(assuming the contract is correct)
at: /home/epi/snippets/a.rkt:3.18
context...:
/usr/share/racket/collects/racket/contract/private/blame.rkt:347:0: 
raise-blame-error
/usr/share/racket/collects/racket/contract/private/arrow-higher-order.rkt:379:33
body of "/home/dan/snippets/blameme.rkt"

So, I am a bit surprised that the error message blames the file a.rkt.
What am I missing here?

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Re: [racket-users] Werid contract violation blame erros

[Alex Harsányi]

I think "define/contract" protects a function from other incorrect calls 
from the same module.  So your "define/provide/contract" is not really 
"define/contract" + "provide".  Here is an example illustrating the 
problem, where the call to "bar" correctly reports the contract violation, 
but the call to "foo" does not:

#lang racket
(define-syntax (define/provide/contract stx)
  (syntax-case stx ()
((_ (name args ...)
contract
body ...)
 #'(begin
 (provide (contract-out (name contract)))
 (define (name args ...)
   body ...)

(define/provide/contract (foo a)
  (-> integer? any/c)
  (printf "a = ~a~%" a))

(define/contract (bar a)
  (-> integer? any/c)
  (printf "a = ~a~%" a))

(foo "Hello")
(bar "Hello")

On Thursday, April 8, 2021 at 4:47:39 AM UTC+8 dominik@trustica.cz 
wrote:

> It is worth noting that it is relatively easy to implement a
> define/provide/contract syntax without any problems. In my two most
> active projects I use it extensively. In one case even with
> scribble/srcdoc to keep the contracts, scribblings and implementation in
> one place.
>
> The simplest version:
>
> (define-syntax (define/provide/contract stx)
> (syntax-case stx ()
> ((_ (name args ...)
> contract
> body ...)
> #'(begin
> (provide (contract-out (name contract)))
> (define (name args ...)
> body ...)
>
> Of course, this is not the best approach for all situations (and I do
> not use it everywhere), but it covers many common use-cases well.
>
>
> Dominik
>
> On 07. 04. 21 22:34, Robby Findler wrote:
> > The short answer: you probably should use (provide (contract-out))
> > instead of define/contract.
> > 
> > The slightly longer answer: when you write a contract, you are not just
> > describing what the legal inputs and outputs are, you are also
> > establishing a *boundary* between two regions of code. In the case of
> > define/contract, you are establishing a boundary between the function
> > (here: f) and the module that it contains (here the file "a.rkt"). In
> > the case of (provide (contract-out  ...)) the boundary is between a.rkt
> > and b.rkt.
> > 
> > The slightly uncomfortable answer: it is my (not completely solid yet)
> > opinion that the boundary that is drawn for define/contract is perhaps
> > not the right / best one. In a theoretical/mathematical sense it is a
> > completely fine and completely defensible one. But it trips people up
> > sometimes. (There are examples others have posted in the past that are
> > perhaps even stranger than yours but boil down to the same specific
> > design choice for define/contract.)
> > 
> > Robby
> > 
> > 
> > 
> > On Wed, Apr 7, 2021 at 2:10 PM epi via Racket Users
> > mailto:racket...@googlegroups.com>>
> > wrote:
> > 
> > Hello Racket users,
> > 
> > I am trying to understand a contract violation message that I am
> > getting.
> > Here is the file a.rkt:
> > 
> > #lang racket
> > (provide f)
> > (define/contract (f a)
> >   (-> boolean? any/c)
> >   '())
> > 
> > and this is b.rkt:
> > 
> > #lang racket
> > (require "a.rkt")
> > (f 3)
> > 
> > 
> > I would expect that the caller is blamed for the contract violation,
> > but the error message that I get is as follows:
> > 
> > 
> > f: contract violation
> >   expected: boolean?
> >   given: 3
> >   in: the 1st argument of
> >   (-> boolean? any/c)
> >   contract from: (function f)
> >   blaming: /home/epi/snippets/a.rkt
> >(assuming the contract is correct)
> >   at: /home/epi/snippets/a.rkt:3.18
> >   context...:
> >  
> >  /usr/share/racket/collects/racket/contract/private/blame.rkt:347:0:
> > raise-blame-error
> >  
> > 
>  
> /usr/share/racket/collects/racket/contract/private/arrow-higher-order.rkt:379:33
> >body of "/home/dan/snippets/blameme.rkt"
> > 
> > So, I am a bit surprised that the error message blames the file a.rkt.
> > What am I missing here?
> > 
> > -- 
> > You received this message because you are subscribed to the Google
> > Groups "Racket Users" group.
> > To unsubscribe from this group and stop receiving emails from it,
> > send an email to racket-users...@googlegroups.com
> > .
> > To view this discussion on the web visit
> > 
> https://groups.google.com/d/msgid/racket-users/149cfc632cd666ff1a92177dce90296b%40disroot.org
> > <
> https://groups.google.com/d/msgid/racket-users/149cfc632cd666ff1a92177dce90296b%40disroot.org
> >.
> > 
> > -- 
> > You received this message because you are subscribed to the Google
> > Groups "Racket Users" group.
> > To unsubscribe from this group and stop receiving emails from it, send
> > an email to racket-users...@googlegroups.com
> > .
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> > 
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> > <
> 

Re: [racket-users] Werid contract violation blame erros

[Robby Findler]

The idea is that a contract violation is not merely telling you "oh,
someone said that you should get an integer? here and you didn't." That is
what an assert macro might do. But contracts offer more.

Contracts are also giving you information to help you hone in on where the
error actually is in your code. The rough idea is that you think of your
program as partitioned into a bunch of pieces. And then contracts "stand
guard" between those pieces, attempting to stop bogus stuff from crossing
over from one piece to another piece. (Simplest form of "piece" for Racket:
a file (containing a module).) So when you put a boundary between `f` and
its containing module, you station guards there -- they tell you "something
bogus came out of f" or "something bogus went into f". For the former/first
one of those errors, it is clear: f is wrong. For the second, however, we
have to decide -- who is this guard standing between? In this code, when we
use define/contract, the other side of the boundary is the rest of the
module containing f. But then, you say -- what happened with b.rkt?! Well,
there was no guard on that boundary! So a.rkt didn't put anything to guard
that boundary, so no checks happened and off the value went -- but now,
hiding inside that value is the guard, and that guard knows only that
boundary it started on, so when that bad input comes in, it blames a.rkt.
a.rkt _could_ have protected itself (using contract-out to station a guard
on the boundary between a.rkt and b.rkt, but it didn't --coulda shoulda
woulda).

Anyway, that's the original idea, dating back to about 2002. More recently,
Christos Dimoulas together with Lukas Lazarek has started an ambitious
research program to try to actually verify that this way of erecting
boundaries actually helps the programmer. Here's a paper on the topic if
you're interested: https://dl.acm.org/doi/10.1145/3371133

Robby


On Wed, Apr 7, 2021 at 6:28 PM Raoul Duke  wrote:

> Clueless newb here. Wait, why can't we have both? As a joe programmer
> on the street I would want the blame to be on b.rkt, and also on any
> function calling f() incorrectly from inside a.rkt. Reading this
> thread it sounds to me like that's not easily available?
>
> On Wed, Apr 7, 2021 at 4:22 PM Robby Findler 
> wrote:
> >
> > No, I don't think it is a bad move if that's your goal! (I usually work
> at the file-level granularity but different code calls for different
> things.) I inferred from epi's message that that wasn't what was going on
> (perhaps incorrectly).
> >
> > Robby
> >
> > On Wed, Apr 7, 2021 at 4:37 PM David Storrs 
> wrote:
> >>
> >> I've always liked define/contract because it guarantees the safety of
> >> the function from erroneous calls by other functions in the module,
> >> which helps with debugging and testing.  It sounds like you think
> >> that's a bad move?
> >>
> >> On Wed, Apr 7, 2021 at 4:35 PM Robby Findler 
> wrote:
> >> >
> >> > The short answer: you probably should use (provide
> (contract-out)) instead of define/contract.
> >> >
> >> > The slightly longer answer: when you write a contract, you are not
> just describing what the legal inputs and outputs are, you are also
> establishing a *boundary* between two regions of code. In the case of
> define/contract, you are establishing a boundary between the function
> (here: f) and the module that it contains (here the file "a.rkt"). In the
> case of (provide (contract-out  ...)) the boundary is between a.rkt and
> b.rkt.
> >> >
> >> > The slightly uncomfortable answer: it is my (not completely solid
> yet) opinion that the boundary that is drawn for define/contract is perhaps
> not the right / best one. In a theoretical/mathematical sense it is a
> completely fine and completely defensible one. But it trips people up
> sometimes. (There are examples others have posted in the past that are
> perhaps even stranger than yours but boil down to the same specific design
> choice for define/contract.)
> >> >
> >> > Robby
> >> >
> >> >
> >> >
> >> > On Wed, Apr 7, 2021 at 2:10 PM epi via Racket Users <
> racket-users@googlegroups.com> wrote:
> >> >>
> >> >> Hello Racket users,
> >> >>
> >> >> I am trying to understand a contract violation message that I am
> getting.
> >> >> Here is the file a.rkt:
> >> >>
> >> >> #lang racket
> >> >> (provide f)
> >> >> (define/contract (f a)
> >> >>   (-> boolean? any/c)
> >> >>   '())
> >> >>
> >> >> and this is b.rkt:
> >> >>
> >> >> #lang racket
> >> >> (require "a.rkt")
> >> >> (f 3)
> >> >>
> >> >>
> >> >> I would expect that the caller is blamed for the contract violation,
> but the error message that I get is as follows:
> >> >>
> >> >>
> >> >> f: contract violation
> >> >>   expected: boolean?
> >> >>   given: 3
> >> >>   in: the 1st argument of
> >> >>   (-> boolean? any/c)
> >> >>   contract from: (function f)
> >> >>   blaming: /home/epi/snippets/a.rkt
> >> >>(assuming the contract is correct)
> >> >>   at: 

Re: [racket-users] Werid contract violation blame erros

[Raoul Duke]

Clueless newb here. Wait, why can't we have both? As a joe programmer
on the street I would want the blame to be on b.rkt, and also on any
function calling f() incorrectly from inside a.rkt. Reading this
thread it sounds to me like that's not easily available?

On Wed, Apr 7, 2021 at 4:22 PM Robby Findler  wrote:
>
> No, I don't think it is a bad move if that's your goal! (I usually work at 
> the file-level granularity but different code calls for different things.) I 
> inferred from epi's message that that wasn't what was going on (perhaps 
> incorrectly).
>
> Robby
>
> On Wed, Apr 7, 2021 at 4:37 PM David Storrs  wrote:
>>
>> I've always liked define/contract because it guarantees the safety of
>> the function from erroneous calls by other functions in the module,
>> which helps with debugging and testing.  It sounds like you think
>> that's a bad move?
>>
>> On Wed, Apr 7, 2021 at 4:35 PM Robby Findler  
>> wrote:
>> >
>> > The short answer: you probably should use (provide (contract-out)) 
>> > instead of define/contract.
>> >
>> > The slightly longer answer: when you write a contract, you are not just 
>> > describing what the legal inputs and outputs are, you are also 
>> > establishing a *boundary* between two regions of code. In the case of 
>> > define/contract, you are establishing a boundary between the function 
>> > (here: f) and the module that it contains (here the file "a.rkt"). In the 
>> > case of (provide (contract-out  ...)) the boundary is between a.rkt and 
>> > b.rkt.
>> >
>> > The slightly uncomfortable answer: it is my (not completely solid yet) 
>> > opinion that the boundary that is drawn for define/contract is perhaps not 
>> > the right / best one. In a theoretical/mathematical sense it is a 
>> > completely fine and completely defensible one. But it trips people up 
>> > sometimes. (There are examples others have posted in the past that are 
>> > perhaps even stranger than yours but boil down to the same specific design 
>> > choice for define/contract.)
>> >
>> > Robby
>> >
>> >
>> >
>> > On Wed, Apr 7, 2021 at 2:10 PM epi via Racket Users 
>> >  wrote:
>> >>
>> >> Hello Racket users,
>> >>
>> >> I am trying to understand a contract violation message that I am getting.
>> >> Here is the file a.rkt:
>> >>
>> >> #lang racket
>> >> (provide f)
>> >> (define/contract (f a)
>> >>   (-> boolean? any/c)
>> >>   '())
>> >>
>> >> and this is b.rkt:
>> >>
>> >> #lang racket
>> >> (require "a.rkt")
>> >> (f 3)
>> >>
>> >>
>> >> I would expect that the caller is blamed for the contract violation, but 
>> >> the error message that I get is as follows:
>> >>
>> >>
>> >> f: contract violation
>> >>   expected: boolean?
>> >>   given: 3
>> >>   in: the 1st argument of
>> >>   (-> boolean? any/c)
>> >>   contract from: (function f)
>> >>   blaming: /home/epi/snippets/a.rkt
>> >>(assuming the contract is correct)
>> >>   at: /home/epi/snippets/a.rkt:3.18
>> >>   context...:
>> >>/usr/share/racket/collects/racket/contract/private/blame.rkt:347:0: 
>> >> raise-blame-error
>> >>
>> >> /usr/share/racket/collects/racket/contract/private/arrow-higher-order.rkt:379:33
>> >>body of "/home/dan/snippets/blameme.rkt"
>> >>
>> >> So, I am a bit surprised that the error message blames the file a.rkt.
>> >> What am I missing here?
>> >>
>> >> --
>> >> You received this message because you are subscribed to the Google Groups 
>> >> "Racket Users" group.
>> >> To unsubscribe from this group and stop receiving emails from it, send an 
>> >> email to racket-users+unsubscr...@googlegroups.com.
>> >> To view this discussion on the web visit 
>> >> https://groups.google.com/d/msgid/racket-users/149cfc632cd666ff1a92177dce90296b%40disroot.org.
>> >
>> > --
>> > You received this message because you are subscribed to the Google Groups 
>> > "Racket Users" group.
>> > To unsubscribe from this group and stop receiving emails from it, send an 
>> > email to racket-users+unsubscr...@googlegroups.com.
>> > To view this discussion on the web visit 
>> > https://groups.google.com/d/msgid/racket-users/CAL3TdONLMx%3Dy_cgfDsY_k9L9yaX_touO52phiK9scziW_jGrOA%40mail.gmail.com.
>>
>> --
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>
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Re: [racket-users] Werid contract violation blame erros

[Robby Findler]

No, I don't think it is a bad move if that's your goal! (I usually work at
the file-level granularity but different code calls for different things.)
I inferred from epi's message that that wasn't what was going on (perhaps
incorrectly).

Robby

On Wed, Apr 7, 2021 at 4:37 PM David Storrs  wrote:

> I've always liked define/contract because it guarantees the safety of
> the function from erroneous calls by other functions in the module,
> which helps with debugging and testing.  It sounds like you think
> that's a bad move?
>
> On Wed, Apr 7, 2021 at 4:35 PM Robby Findler 
> wrote:
> >
> > The short answer: you probably should use (provide (contract-out))
> instead of define/contract.
> >
> > The slightly longer answer: when you write a contract, you are not just
> describing what the legal inputs and outputs are, you are also establishing
> a *boundary* between two regions of code. In the case of define/contract,
> you are establishing a boundary between the function (here: f) and the
> module that it contains (here the file "a.rkt"). In the case of (provide
> (contract-out  ...)) the boundary is between a.rkt and b.rkt.
> >
> > The slightly uncomfortable answer: it is my (not completely solid yet)
> opinion that the boundary that is drawn for define/contract is perhaps not
> the right / best one. In a theoretical/mathematical sense it is a
> completely fine and completely defensible one. But it trips people up
> sometimes. (There are examples others have posted in the past that are
> perhaps even stranger than yours but boil down to the same specific design
> choice for define/contract.)
> >
> > Robby
> >
> >
> >
> > On Wed, Apr 7, 2021 at 2:10 PM epi via Racket Users <
> racket-users@googlegroups.com> wrote:
> >>
> >> Hello Racket users,
> >>
> >> I am trying to understand a contract violation message that I am
> getting.
> >> Here is the file a.rkt:
> >>
> >> #lang racket
> >> (provide f)
> >> (define/contract (f a)
> >>   (-> boolean? any/c)
> >>   '())
> >>
> >> and this is b.rkt:
> >>
> >> #lang racket
> >> (require "a.rkt")
> >> (f 3)
> >>
> >>
> >> I would expect that the caller is blamed for the contract violation,
> but the error message that I get is as follows:
> >>
> >>
> >> f: contract violation
> >>   expected: boolean?
> >>   given: 3
> >>   in: the 1st argument of
> >>   (-> boolean? any/c)
> >>   contract from: (function f)
> >>   blaming: /home/epi/snippets/a.rkt
> >>(assuming the contract is correct)
> >>   at: /home/epi/snippets/a.rkt:3.18
> >>   context...:
> >>/usr/share/racket/collects/racket/contract/private/blame.rkt:347:0:
> raise-blame-error
> >>
> /usr/share/racket/collects/racket/contract/private/arrow-higher-order.rkt:379:33
> >>body of "/home/dan/snippets/blameme.rkt"
> >>
> >> So, I am a bit surprised that the error message blames the file a.rkt.
> >> What am I missing here?
> >>
> >> --
> >> You received this message because you are subscribed to the Google
> Groups "Racket Users" group.
> >> To unsubscribe from this group and stop receiving emails from it, send
> an email to racket-users+unsubscr...@googlegroups.com.
> >> To view this discussion on the web visit
> https://groups.google.com/d/msgid/racket-users/149cfc632cd666ff1a92177dce90296b%40disroot.org
> .
> >
> > --
> > You received this message because you are subscribed to the Google
> Groups "Racket Users" group.
> > To unsubscribe from this group and stop receiving emails from it, send
> an email to racket-users+unsubscr...@googlegroups.com.
> > To view this discussion on the web visit
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> .
>
> --
> You received this message because you are subscribed to the Google Groups
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> .
>

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Re: [racket-users] Werid contract violation blame erros

[David Storrs]

I've always liked define/contract because it guarantees the safety of
the function from erroneous calls by other functions in the module,
which helps with debugging and testing.  It sounds like you think
that's a bad move?

On Wed, Apr 7, 2021 at 4:35 PM Robby Findler  wrote:
>
> The short answer: you probably should use (provide (contract-out)) 
> instead of define/contract.
>
> The slightly longer answer: when you write a contract, you are not just 
> describing what the legal inputs and outputs are, you are also establishing a 
> *boundary* between two regions of code. In the case of define/contract, you 
> are establishing a boundary between the function (here: f) and the module 
> that it contains (here the file "a.rkt"). In the case of (provide 
> (contract-out  ...)) the boundary is between a.rkt and b.rkt.
>
> The slightly uncomfortable answer: it is my (not completely solid yet) 
> opinion that the boundary that is drawn for define/contract is perhaps not 
> the right / best one. In a theoretical/mathematical sense it is a completely 
> fine and completely defensible one. But it trips people up sometimes. (There 
> are examples others have posted in the past that are perhaps even stranger 
> than yours but boil down to the same specific design choice for 
> define/contract.)
>
> Robby
>
>
>
> On Wed, Apr 7, 2021 at 2:10 PM epi via Racket Users 
>  wrote:
>>
>> Hello Racket users,
>>
>> I am trying to understand a contract violation message that I am getting.
>> Here is the file a.rkt:
>>
>> #lang racket
>> (provide f)
>> (define/contract (f a)
>>   (-> boolean? any/c)
>>   '())
>>
>> and this is b.rkt:
>>
>> #lang racket
>> (require "a.rkt")
>> (f 3)
>>
>>
>> I would expect that the caller is blamed for the contract violation, but the 
>> error message that I get is as follows:
>>
>>
>> f: contract violation
>>   expected: boolean?
>>   given: 3
>>   in: the 1st argument of
>>   (-> boolean? any/c)
>>   contract from: (function f)
>>   blaming: /home/epi/snippets/a.rkt
>>(assuming the contract is correct)
>>   at: /home/epi/snippets/a.rkt:3.18
>>   context...:
>>/usr/share/racket/collects/racket/contract/private/blame.rkt:347:0: 
>> raise-blame-error
>>
>> /usr/share/racket/collects/racket/contract/private/arrow-higher-order.rkt:379:33
>>body of "/home/dan/snippets/blameme.rkt"
>>
>> So, I am a bit surprised that the error message blames the file a.rkt.
>> What am I missing here?
>>
>> --
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>> "Racket Users" group.
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>
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Re: [racket-users] Werid contract violation blame erros

[Dominik Pantůček]

It is worth noting that it is relatively easy to implement a
define/provide/contract syntax without any problems. In my two most
active projects I use it extensively. In one case even with
scribble/srcdoc to keep the contracts, scribblings and implementation in
one place.

The simplest version:

(define-syntax (define/provide/contract stx)
  (syntax-case stx ()
((_ (name args ...)
contract
body ...)
 #'(begin
 (provide (contract-out (name contract)))
 (define (name args ...)
   body ...)

Of course, this is not the best approach for all situations (and I do
not use it everywhere), but it covers many common use-cases well.


Dominik

On 07. 04. 21 22:34, Robby Findler wrote:
> The short answer: you probably should use (provide (contract-out))
> instead of define/contract.
> 
> The slightly longer answer: when you write a contract, you are not just
> describing what the legal inputs and outputs are, you are also
> establishing a *boundary* between two regions of code. In the case of
> define/contract, you are establishing a boundary between the function
> (here: f) and the module that it contains (here the file "a.rkt"). In
> the case of (provide (contract-out  ...)) the boundary is between a.rkt
> and b.rkt.
> 
> The slightly uncomfortable answer: it is my (not completely solid yet)
> opinion that the boundary that is drawn for define/contract is perhaps
> not the right / best one. In a theoretical/mathematical sense it is a
> completely fine and completely defensible one. But it trips people up
> sometimes. (There are examples others have posted in the past that are
> perhaps even stranger than yours but boil down to the same specific
> design choice for define/contract.)
> 
> Robby
> 
> 
> 
> On Wed, Apr 7, 2021 at 2:10 PM epi via Racket Users
> mailto:racket-users@googlegroups.com>>
> wrote:
> 
> Hello Racket users,
> 
> I am trying to understand a contract violation message that I am
> getting.
> Here is the file a.rkt:
> 
>     #lang racket
>     (provide f)
>     (define/contract (f a)
>       (-> boolean? any/c)
>       '())
> 
> and this is b.rkt:
> 
>     #lang racket
>     (require "a.rkt")
>     (f 3)
> 
> 
> I would expect that the caller is blamed for the contract violation,
> but the error message that I get is as follows:
> 
> 
> f: contract violation
>   expected: boolean?
>   given: 3
>   in: the 1st argument of
>       (-> boolean? any/c)
>   contract from: (function f)
>   blaming: /home/epi/snippets/a.rkt
>    (assuming the contract is correct)
>   at: /home/epi/snippets/a.rkt:3.18
>   context...:
>  
>  /usr/share/racket/collects/racket/contract/private/blame.rkt:347:0:
> raise-blame-error
>  
>  
> /usr/share/racket/collects/racket/contract/private/arrow-higher-order.rkt:379:33
>    body of "/home/dan/snippets/blameme.rkt"
> 
> So, I am a bit surprised that the error message blames the file a.rkt.
> What am I missing here?
> 
> -- 
> You received this message because you are subscribed to the Google
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> 
> https://groups.google.com/d/msgid/racket-users/149cfc632cd666ff1a92177dce90296b%40disroot.org
> 
> .
> 
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> .

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Re: [racket-users] Werid contract violation blame erros

[Robby Findler]

The short answer: you probably should use (provide (contract-out))
instead of define/contract.

The slightly longer answer: when you write a contract, you are not just
describing what the legal inputs and outputs are, you are also establishing
a *boundary* between two regions of code. In the case of define/contract,
you are establishing a boundary between the function (here: f) and the
module that it contains (here the file "a.rkt"). In the case of (provide
(contract-out  ...)) the boundary is between a.rkt and b.rkt.

The slightly uncomfortable answer: it is my (not completely solid yet)
opinion that the boundary that is drawn for define/contract is perhaps not
the right / best one. In a theoretical/mathematical sense it is a
completely fine and completely defensible one. But it trips people up
sometimes. (There are examples others have posted in the past that are
perhaps even stranger than yours but boil down to the same specific design
choice for define/contract.)

Robby



On Wed, Apr 7, 2021 at 2:10 PM epi via Racket Users <
racket-users@googlegroups.com> wrote:

> Hello Racket users,
>
> I am trying to understand a contract violation message that I am getting.
> Here is the file a.rkt:
>
> #lang racket
> (provide f)
> (define/contract (f a)
>   (-> boolean? any/c)
>   '())
>
> and this is b.rkt:
>
> #lang racket
> (require "a.rkt")
> (f 3)
>
>
> I would expect that the caller is blamed for the contract violation, but
> the error message that I get is as follows:
>
>
> f: contract violation
>   expected: boolean?
>   given: 3
>   in: the 1st argument of
>   (-> boolean? any/c)
>   contract from: (function f)
>   blaming: /home/epi/snippets/a.rkt
>(assuming the contract is correct)
>   at: /home/epi/snippets/a.rkt:3.18
>   context...:
>/usr/share/racket/collects/racket/contract/private/blame.rkt:347:0:
> raise-blame-error
>
>  
> /usr/share/racket/collects/racket/contract/private/arrow-higher-order.rkt:379:33
>body of "/home/dan/snippets/blameme.rkt"
>
> So, I am a bit surprised that the error message blames the file a.rkt.
> What am I missing here?
>
> --
> You received this message because you are subscribed to the Google Groups
> "Racket Users" group.
> To unsubscribe from this group and stop receiving emails from it, send an
> email to racket-users+unsubscr...@googlegroups.com.
> To view this discussion on the web visit
> https://groups.google.com/d/msgid/racket-users/149cfc632cd666ff1a92177dce90296b%40disroot.org
> .
>

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[racket-users] Werid contract violation blame erros

[epi via Racket Users]

Hello Racket users,

I am trying to understand a contract violation message that I am getting.
Here is the file a.rkt:

#lang racket
(provide f)
(define/contract (f a)
  (-> boolean? any/c)
  '())

and this is b.rkt:

#lang racket
(require "a.rkt")
(f 3)


I would expect that the caller is blamed for the contract violation, but the 
error message that I get is as follows:


f: contract violation
  expected: boolean?
  given: 3
  in: the 1st argument of
  (-> boolean? any/c)
  contract from: (function f)
  blaming: /home/epi/snippets/a.rkt
   (assuming the contract is correct)
  at: /home/epi/snippets/a.rkt:3.18
  context...:
   /usr/share/racket/collects/racket/contract/private/blame.rkt:347:0: 
raise-blame-error
   
/usr/share/racket/collects/racket/contract/private/arrow-higher-order.rkt:379:33
   body of "/home/dan/snippets/blameme.rkt"

So, I am a bit surprised that the error message blames the file a.rkt.
What am I missing here?

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