Adding if-let, when-let, cond-let, etc? #1

New Issue

2016-07-28T14:20:20+01:00

AlexKnauth commented

2016-07-28 14:20:20 +01:00

(Migrated from github.com)

Should hygienic versions of these anaphoric macros be provided as well, where the user would specify an identifier to bind?

(if-let [x (member 'a lst)]
  (displayln x)
  (displayln "not found"))

(when-let [x (member 'a lst)]
  (displayln x))

(cond-let
  [[x (member 'a lst)] (displayln x)]
  [[x (member 'b lst)] (displayln x)]
  [else (displayln "not found")])

The syntax-parameter versions, aif, awhen, and acond, would have straightforward translations into these + syntax-parameterize.

(define-syntax-rule (aif condition true-branch false-branch)
  (if-let [tmp condition]
    (syntax-parameterize ([it (make-rename-transformer #'tmp)])
      true-branch)
    false-branch))

(define-syntax-rule (awhen condition . body)
  (when-let [tmp condition]
    (syntax-parameterize ([it (make-rename-transformer #'tmp)])
      . body)))

(define-syntax acond
  (syntax-parser #:literals (else)
    [(acond [condition . body] ... [else . else-body])
     #'(cond-let
         [[tmp condition]
          (syntax-parameterize ([it (make-rename-transformer #'tmp)])
            . body)]
         ...
         [else . else-body])]
    [(acond [condition . body] ...)
     #'(cond-let
         [[tmp condition]
          (syntax-parameterize ([it (make-rename-transformer #'tmp)])
            . body)]
         ...)]

Should hygienic versions of these anaphoric macros be provided as well, where the user would specify an identifier to bind? ``` racket (if-let [x (member 'a lst)] (displayln x) (displayln "not found")) ``` ``` racket (when-let [x (member 'a lst)] (displayln x)) ``` ``` racket (cond-let [[x (member 'a lst)] (displayln x)] [[x (member 'b lst)] (displayln x)] [else (displayln "not found")]) ``` The syntax-parameter versions, `aif`, `awhen`, and `acond`, would have straightforward translations into these + `syntax-parameterize`. ``` racket (define-syntax-rule (aif condition true-branch false-branch) (if-let [tmp condition] (syntax-parameterize ([it (make-rename-transformer #'tmp)]) true-branch) false-branch)) ``` ``` racket (define-syntax-rule (awhen condition . body) (when-let [tmp condition] (syntax-parameterize ([it (make-rename-transformer #'tmp)]) . body))) ``` ``` racket (define-syntax acond (syntax-parser #:literals (else) [(acond [condition . body] ... [else . else-body]) #'(cond-let [[tmp condition] (syntax-parameterize ([it (make-rename-transformer #'tmp)]) . body)] ... [else . else-body])] [(acond [condition . body] ...) #'(cond-let [[tmp condition] (syntax-parameterize ([it (make-rename-transformer #'tmp)]) . body)] ...)] ```

SuzanneSoy commented

2016-08-08 11:48:18 +01:00

(Migrated from github.com)

Sorry about the delay, for some reason GitHub didn't notify me about this issue. I added the if-let, when-let and cond-let (with a shorthand for cond-let where you need specify only one identifier). I didn't change the implementation of aif, awhen and acond, though, as it was rather short and simple anyway.

Sorry about the delay, for some reason GitHub didn't notify me about this issue. I added the `if-let`, `when-let` and `cond-let` (with a shorthand for `cond-let` where you need specify only one identifier). I didn't change the implementation of `aif`, `awhen` and `acond`, though, as it was rather short and simple anyway.

AlexKnauth commented

2016-08-08 14:56:56 +01:00

(Migrated from github.com)

They wouldn't be shorter and simpler in terms of the -let versions?

They wouldn't be shorter and simpler in terms of the `-let` versions?

SuzanneSoy commented

2016-08-08 15:10:17 +01:00

(Migrated from github.com)

@AlexKnauth The code you suggested is shorter by one line, and looks nearly the same ((let ([tmp c]) (if …)) instead of (if-let (tmp c) …)), so no big deal. Also, the extra level of indirection wouldn't improve readability. I did reuse part of your acond code to implement cond-let, though :) .

@AlexKnauth The code you suggested is shorter by one line, and looks nearly the same (`(let ([tmp c]) (if …))` instead of `(if-let (tmp c) …)`), so no big deal. Also, the extra level of indirection wouldn't improve readability. I did reuse part of your acond code to implement cond-let, though :) .

AlexKnauth commented

2016-08-08 15:16:25 +01:00

(Migrated from github.com)

Okay

SuzanneSoy commented

2017-01-19 17:54:40 +00:00

(Migrated from github.com)

Hi @AlexKnauth ! In order to match the current Racket relicensing, I'm changing the license for all my project to LGPL + MIT + Apache 2 + CC0 (to cover any potential future license change).

Since you contributed some code, is that okay for you concerning this project?

Thanks,
Georges Dupéron

Hi @AlexKnauth ! In order to match the current Racket relicensing, I'm changing the license for all my project to LGPL + MIT + Apache 2 + CC0 (to cover any potential future license change). Since you contributed some code, is that okay for you concerning this project? Thanks, Georges Dupéron

AlexKnauth commented

2017-01-19 21:41:59 +00:00

(Migrated from github.com)

Yes.

SuzanneSoy commented

2017-01-19 21:42:53 +00:00

(Migrated from github.com)

Thanks!

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