Re: The `if-let` construct

[Rick Hanson]

A subtle change in the definition.  Added two quote marks.

(de if-let Args
  ## Better than anaphoric `if` because you can name the test result
  ## yourself.
  (let ((@Pattern Test @Then @Else) Args
@Test-Result (eval Test))
(eval
 (fill
  '(if '@Test-Result
   (let (@Pattern '@Test-Result)
 @Then)
   @Else)
  '(@Pattern @Test-Result @Then @Else)
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Re: The `if-let` construct

[Mike Pechkin]

hi all,

I totally dont like it. I vote.
All this useless.
Can you write here #Usage examples of (if-let) in current language feature
set?
Coder already have all needed, like in forth.
If coder want, it have all required to implement this by himself.
Like reduce and - from clojure I've used to emulate for 4clojure tasks.

(de reduce (Fun Lst)
   (let A (car Lst)
  (for N (cdr Lst)
 (setq A (Fun A N)) )
  A ) )
(de - (X . A)
   (for Form A
  (setq X (apply (car Form) (cdr Form) X)) ) )


Code is ok, but useless inside pl feature set.

p.s. Create wiki page and collect code as useful snippets.





On Mon, Aug 10, 2015 at 5:10 AM, Rick Hanson cryptor...@gmail.com wrote:

 I like the idea of `if-let` (which you can see in Clojure[1], and
 probably other languages).  It's like anaphoric `if`[2] but better
 because you can name the result of the conditional test.  (Intentional
 variable capture is always best when you are controlling the variable
 names; not having a name forced upon you.)

 I think this is the idea Alex had for `let?`[3]; however it seems
 `let?` doesn't work with pil64 pattern bindings (i.e. destructuring
 binds) and `let?` doesn't seem to have the possibility of an else
 clause (either that or my imagination and overall pil knowledge
 suffers enough not to see such a possibility -- Alex will tell me :).

 Anyway, please feel comment, criticize, etc.  If you do, I will learn
 something.  Thanks!  --Rick

 # Usages
 (if-let X 13 (- X 1) 0)   #- 12
 (if-let X 13 (- X 1)) #- 12  # you can leave off the Else clause.
 (if-let X '() (- X 1))#- NIL # b/c no Else clause and false condition.
 (if-let X 0 (+ 42 X) -1)  #- 42  # because 0 is truthy.
 (if-let (X Y) (1 2) Y 42) #- 2   # pattern binding for pil64.
 (if-let (X Y) 1 Y 42) #- NIL # because the pattern binding failed.
 (if-let (X Y) '() Y test failed) #- test failed
 (if-let (X Y) NIL Y test failed) #- test failed

 # A definition
 (de if-let Args
   ## Better than anaphoric `if` because you can name the test result
   ## yourself.
   (let ((@Pattern Test @Then @Else) Args
 @Test-Result (eval Test))
 (eval
  (fill
   '(if @Test-Result
(let (@Pattern @Test-Result)
  @Then)
@Else)
   '(@Pattern @Test-Result @Then @Else)

 
 [1] http://clojuredocs.org/clojure.core/if-let
 [2] http://www.bookshelf.jp/texi/onlisp/onlisp_15.html#SEC100
 [3] http://software-lab.de/doc/refL.html#let?
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Re: The `if-let` construct

[Alexander Burger]

Hi Rick,

thanks for the ideas :)

However, as Mike Pechkin wrote, I do also believe that such a construct
is rather useless.

The examples are too simple. They can be easily rewritten as:

 (if-let X 13 (- X 1) 0)   #- 12

   (if 13 (dec @) 0)

 (if-let X 13 (- X 1)) #- 12  # you can leave off the Else clause.

   (if 13 (dec @))

 (if-let X 0 (+ 42 X) -1)  #- 42  # because 0 is truthy.

   (if 0 (+ @ 42) -1)

and so on ..

Can you provide an example where it is shorter with 'if-let' than with
existing flow functions?

Note that you can always do

   (if (condition)
  (let X @
 (do-something 1) )
  (do-something 2)
  (do-something 3) )


 # A definition
 (de if-let Args
   ## Better than anaphoric `if` because you can name the test result
   ## yourself.
   (let ((@Pattern Test @Then @Else) Args
 @Test-Result (eval Test))
 (eval
  (fill
   '(if @Test-Result
(let (@Pattern @Test-Result)
  @Then)
@Else)
   '(@Pattern @Test-Result @Then @Else)

This is a very un-PicoLisp-ish solution. It is a huge overhead to
rebuild the whole expression with 'fill' each time it is called.

Instead, you need to interprete it directly:

   (de if-let Args
  (if (eval (cadr Args))
 (bind (car Args)
(set (car Args) @)
(eval  (caddr Args)) )
 (run (cdddr Args)) ) )

   : (if-let X (* 3 4) (inc X) Nope)
   - 13

   : (if-let X NIL (inc X) Nope)
   - Nope

♪♫ Alex
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