Re: [racket-users] Expression context

[Leif Andersen]

Oh cool, I didn't know that existed. Thanks for sharing.


~Leif Andersen

On Tue, Sep 15, 2015 at 12:37 AM, Jens Axel Søgaard 
wrote:

> FWIW an alternative to (let () ...) is block.
>
> http://docs.racket-lang.org/reference/block.html?q=block
>
> 2015-09-15 0:16 GMT+02:00 John Carmack :
>
>> Is there a deep reason why defines aren’t allowed in the branches of an
>> if, but are for cond / while / unless?
>>
>>
>>
>> Wrapping code in (let() …) instead of (begin …) works fine, but it is a
>> strange quirk to explain to someone else.
>>
>>
>>
>> --
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>
>
>
> --
> --
> Jens Axel Søgaard
>
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Re: [racket-users] Expression context

[Matthias Felleisen]

On Sep 14, 2015, at 6:16 PM, John Carmack  wrote:

> Is there a deep reason why defines aren’t allowed in the branches of an if, 
> but are for cond / while / unless?
>  
> Wrapping code in (let() …) instead of (begin …) works fine, but it is a 
> strange quirk to explain to someone else.


It exposes our history. Traditionally there are no extra markers in LISP-style 
syntax to introduce phrases within a grammatical sentence. Thus, an 
if-expression obeys the grammar 

 (if test-expression 
 then-expression 
 else-expression)

not an Algol-style variant with additional keywords: 

 (if test-expression 
 #:then then-expression 
 #:else else-expression)

If we had those markers (not just in if-expressions but wherever we may allow 
sequences of expressions), we could mixin definitions wherever we wanted. 

In cond, it is obvious which phrase is a branch (due to the 
parentheses/brackets) and that the first part of each branch is a 
test-expression. The extra pair of parens play the role of Algol-style markers. 

Now, if Racket had emerged as an alternative attempt to address all of Lisp's 
weaknesses, we might have overcome this problem, too. But Racket grew 
organically and we value backward compatibility very highly for our 
programmers. You might say we are the C++ of the LISP world, but I hope we can 
do better in the long run. 

-- Matthias

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[racket-users] Expression context

[John Carmack]

Is there a deep reason why defines aren't allowed in the branches of an if, but 
are for cond / while / unless?

Wrapping code in (let() ...) instead of (begin ...) works fine, but it is a 
strange quirk to explain to someone else.

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Re: [racket-users] Expression context

[Leif Andersen]

> Wrapping code in (let() …) instead of (begin …) works fine, but it is a
strange quirk to explain to someone else.

This is because begin actually does not create new scope in racket, where
as (let () ...) does.

Consider the following code:

#lang racket

(define (f x)
  (begin
(define g 5))
  (+ x g))

(f 6) ; => 11

Here, g is still in scope, despite it being defined in a begin. If it was
changed to a (let () ...) then the code would error.

The reason it doesn't make sense to allow for sequencing statements in an
if block is a matter of binding. Consider the following expression:

(if #f
   (begin
  (define x 5)
  x)
   12))

(displaln x) ; ???

Should x be bound outside the if?

There are a few ways to get around this. The first would be to make if
forms create new scope in it's then and else clauses. This is what when and
unless do, and why you can use define in it. If you want to do this you
could with a macro:

(define-syntax-rule (my-if condition then else)
  (if condition (let () then) (let () else)))

(my-if #f
   (begin
 (define x 5)
 x)
   12)

Another is to use a form that creates new scope. (let () ...) does this,
but if you find (let () ...) unappealing, you can create a macro to get rid
of it.

(define-syntax-rule (new-scope . body)
  (let () . body))

(if #f
(new-scope
  (define x 5)
  x)
12)

I hope that makes it more clear why you can use let but not begin to define
variables in an if.



~Leif Andersen

On Mon, Sep 14, 2015 at 6:24 PM, Matthias Felleisen 
wrote:

>
> On Sep 14, 2015, at 6:16 PM, John Carmack  wrote:
>
> > Is there a deep reason why defines aren’t allowed in the branches of an
> if, but are for cond / while / unless?
> >
> > Wrapping code in (let() …) instead of (begin …) works fine, but it is a
> strange quirk to explain to someone else.
>
>
> It exposes our history. Traditionally there are no extra markers in
> LISP-style syntax to introduce phrases within a grammatical sentence. Thus,
> an if-expression obeys the grammar
>
>  (if test-expression
>  then-expression
>  else-expression)
>
> not an Algol-style variant with additional keywords:
>
>  (if test-expression
>  #:then then-expression
>  #:else else-expression)
>
> If we had those markers (not just in if-expressions but wherever we may
> allow sequences of expressions), we could mixin definitions wherever we
> wanted.
>
> In cond, it is obvious which phrase is a branch (due to the
> parentheses/brackets) and that the first part of each branch is a
> test-expression. The extra pair of parens play the role of Algol-style
> markers.
>
> Now, if Racket had emerged as an alternative attempt to address all of
> Lisp's weaknesses, we might have overcome this problem, too. But Racket
> grew organically and we value backward compatibility very highly for our
> programmers. You might say we are the C++ of the LISP world, but I hope we
> can do better in the long run.
>
> -- Matthias
>
> --
> 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.
> For more options, visit https://groups.google.com/d/optout.
>

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Re: [racket-users] Expression context

[Jens Axel Søgaard]

FWIW an alternative to (let () ...) is block.

http://docs.racket-lang.org/reference/block.html?q=block

2015-09-15 0:16 GMT+02:00 John Carmack :

> Is there a deep reason why defines aren’t allowed in the branches of an
> if, but are for cond / while / unless?
>
>
>
> Wrapping code in (let() …) instead of (begin …) works fine, but it is a
> strange quirk to explain to someone else.
>
>
>
> --
> 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.
> For more options, visit https://groups.google.com/d/optout.
>



-- 
-- 
Jens Axel Søgaard

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