Re: [racket-users] What is the Racket equivalent to 'return' for early exit?

[Matthew Butterick]

On Nov 21, 2016, at 9:37 AM, David Storrs  wrote:

> In Perl I would often write:
> 
> sub do_something {
> return unless ( some necessary condition is met );
> ... do the thing ...
> }



On Nov 21, 2016, at 10:07 AM, Leif Andersen  wrote:

> Although honestly, with this pattern, I find that errors work better here, as 
> they return early, and you can decide how they get handled with a 
> with-handlers.
> 
> (define (do-something)
>   (unless (some-condition)
> (error "NO"))
>   (do-the-thing))
> 
> (with-handlers ([exn:fail? (lambda (e) (displayln "I returned early"))])
>   (do-something))


I agree with Leif. I've learned this pattern from studying well-written Racket 
code. 

As I understand it, the policy idea is that when a called function doesn't do 
what it was asked to do, then the caller of the function — not the called 
function — should get to decide how serious a problem it is. So `error` / 
`with-handlers` is the best idiom.

I think of `return` as a way for a function to deliver its *expected result*, 
in the special circumstance where it has to bust out of a deeply nested block. 
IOW, `return` is not for error conditions. 

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Re: [racket-users] What is the Racket equivalent to 'return' for early exit?

[Norman Gray]

Greetings.

On 21 Nov 2016, at 18:07, Leif Andersen wrote:


(define (do-something)
  (unless (some-condition)
(error "NO"))
  (do-the-thing))

(with-handlers ([exn:fail? (lambda (e) (displayln "I returned 
early"))])

  (do-something))

But that is specifically because I prefer the workflow of catching 
errors,
rather than always having to manually check the output of the 
function

(I'm looking at you C...)


On reflection, I think this is a better answer than mine (and is 
probably what I'd do in fact).


Apart from anything else, it means that (do-something) will only return 
the type of thing it's supposed to return, and not anything that needs 
checked.  That would be still more important if you were using Typed 
Racket.


All the best,

Norman


--
Norman Gray  :  https://nxg.me.uk
SUPA School of Physics and Astronomy, University of Glasgow, UK

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Re: [racket-users] What is the Racket equivalent to 'return' for early exit?

[David Storrs]

On Mon, Nov 21, 2016 at 10:30 AM, 'John Clements' via Racket Users <
racket-users@googlegroups.com> wrote:

>
> > On Nov 21, 2016, at 09:37, David Storrs  wrote:
> >
> > In Perl I would often write:
> >
> > sub do_something {
> > return unless ( some necessary condition is met );
> > ... do the thing ...
> > }
> >
> > In Racket I could wrap the rest of the procedure in an (if), but that
> adds an unnecessary level of indentation and feels clunky.  Is there a
> clean solution?
>
> Playing the role of irritating ideologue, it’s … me!
>
> Yes, there’s let/ec, but my (limited) experience in industry suggests that
> reading functions that use ‘return’ liberally can be massively more
> difficult than one that uses ‘if’ or a related form.


I mostly agree with you.  I've found that functions should have at most two
return points:  one at the top and one at the bottom.  The one at the top
is just for checking preconditions before you start doing real work, the
one at the bottom is the happy path result.


> BTW: yes, my soapbox is huge. Huge!
>

I think you meant 'yuje'.  Or perhaps 'bigly'?  ;>



>
> John
>
>
>
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Re: [racket-users] What is the Racket equivalent to 'return' for early exit?

['John Clements' via Racket Users]

> On Nov 21, 2016, at 09:37, David Storrs  wrote:
> 
> In Perl I would often write:
> 
> sub do_something {
> return unless ( some necessary condition is met );
> ... do the thing ...
> }
> 
> In Racket I could wrap the rest of the procedure in an (if), but that adds an 
> unnecessary level of indentation and feels clunky.  Is there a clean solution?

Playing the role of irritating ideologue, it’s … me!

Yes, there’s let/ec, but my (limited) experience in industry suggests that 
reading functions that use ‘return’ liberally can be massively more difficult 
than one that uses ‘if’ or a related form. I would argue that the call/ec 
doesn’t actually make the code less clunky, it just hides the clunkiness, and 
makes the code harder to read, to boot. In the absence of return, you can 
generally easily deduce when control flow reaches a particular point (“we only 
get here if x is > 0 and the string is empty”). In the presence of return, this 
becomes “we only get here if x is > 0 and the string is empty and one of these 
19 incomprehensible clauses didn’t trigger a return.” 

I do think that call/ec makes sense in some circumstances; for instance, if you 
want to bail out of a deeply nested call, and the intervening calls are to 
functions that someone else wrote. But generally, I try to bite the bullet and 
use the if (or cond), on the theory that when I come to read my own code next 
year I’ll have some idea what it’s doing.

BTW: yes, my soapbox is huge. Huge!

John



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[racket-users] What is the Racket equivalent to 'return' for early exit?

[David Storrs]

In Perl I would often write:

sub do_something {
return unless ( some necessary condition is met );
... do the thing ...
}

In Racket I could wrap the rest of the procedure in an (if), but that adds
an unnecessary level of indentation and feels clunky.  Is there a clean
solution?

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