# Macro problem i do not understand

**URL:** <https://racket.discourse.group/t/macro-problem-i-do-not-understand/1864>\
**Category:** General\
**Created:** [April 14, 2023, 7:11pm UTC](https://racket.discourse.group/t/macro-problem-i-do-not-understand/1864 "2023-04-14T19:11:57Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![damien\_mattei](https://yyz2.discourse-cdn.com/free1/user_avatar/racket.discourse.group/damien_mattei/32/2706_2.png) [@damien\_mattei](https://racket.discourse.group/u/damien_mattei)\
**Post date:** [April 14, 2023, 7:11pm UTC](https://racket.discourse.group/t/macro-problem-i-do-not-understand/1864/1 "2023-04-14T19:11:57Z")

</div>

hello,

i have a problem that i tested both with Racket and Guile,

i have 2 macros used in one expression like this:  
scheme@(guile-user)\> (define i 2)  
scheme@(guile-user)\> {i \<- i + 1}  
and i got this error:  
While compiling expression:  
Syntax error:  
unknown location: source expression failed to match any pattern in form \<-

i use SRFI-105 so :  
'{i \<- i + 1} expand in:  
(nfx i \<- i + 1)

and i'm expecting nfx to be called but none of this happens:  
scheme@(guile-user)\> (nfx i \<- i + 1)  
While compiling expression:  
Syntax error:  
unknown location: source expression failed to match any pattern in form \<-

it seems to be the \<- macro and i do not understand why?

any idea?

macros are defined like this for the beginning:  
;; from file assignment.scm  
(define-syntax \<-

(syntax-rules ()  
;; special form like : (\<- (bracket-apply T 3) (bracket-apply T 4))

```
;; one dimension array, example: {a[4] <- 7}
;; $bracket-apply$ is from SRFI 105 bracket-apply is an argument of the macro
((_ (bracket-apply container index) expr)

```

....

;; from file scheme-infix.scm  
(define-syntax nfx  
(syntax-rules ()

```
((_ ident opspecial term1 op term2) (cond ((or (equal? (quote opspecial) (quote <-)) (equal? (quote opspecial) (quote ←)))
  (begin

```

(display "nfx") (newline)  
(opspecial ident (op term1 term2)))) ;; {ident \<- {term1 op term2}}

...

it is in a module like this:

(define-module (Scheme+)

#:use-module (growable-vector)  
#:use-module (srfi srfi-69) ;; Basic hash tables  
#:use-module (srfi srfi-31) ;; rec  
#:export (nfx def bracket-apply \<- ← -\> → \<+ ⥆ +\> ⥅ declare $ & condx \<\> ≠ \*\* ⇜ ⇝ repeat)  
#:replace (do when unless))

(include-from-path "def.scm")  
(include-from-path "array.scm")  
(include-from-path "set-values-plus.scm")  
(include-from-path "apply-square-brackets.scm")  
(include-from-path "assignment.scm")  
(include-from-path "declare.scm")  
(include-from-path "condx.scm")  
(include-from-path "block.scm")  
(include-from-path "not-equal.scm")  
(include-from-path "exponential.scm")  
(include-from-path "while-do-when-unless.scm")  
(include-from-path "repeat-until.scm")  
(include-from-path "scheme-infix.scm")

if it can help.

i have found that the error is related with this :

(define-syntax nfx  
(syntax-rules ()

```
((_ ident opspecial term1 op term2) (cond ((or (equal? (quote opspecial) (quote <-)) (equal? (quote opspecial) (quote ←)))
  (begin

```

(display "nfx") (newline)  
(opspecial ident (op term1 term2)))) ;; {ident \<- term1 op term2}

```
 ((or (equal? (quote op) (quote ->)) (equal? (quote op) (quote →))) (op term2 (opspecial ident term1))) ;; Warning: argument names of macro do not reprensent the values contained in this case

 (else (! ident (quote opspecial) term1 op term2))))

```

this code works:  
scheme@(guile-user)\> (define i 2)  
scheme@(guile-user)\> {i \<- i + 1}  
nfx  
3

but if i change the last line of the else clause by removing the quote of opspecial:  
(else (! ident opspecial term1 op term2))))

it will fail:

scheme@(guile-user)\> {i \<- i + 1}  
While compiling expression:  
Syntax error:  
unknown location: source expression failed to match any pattern in form \<-

i understand the problem is with opspecial equal in this example to the special form \<- but what i do not understand is why the code is then going to the else clause as i know that previously it was on the same example evaluating the first clause:  
(begin  
(display "nfx") (newline)  
(opspecial ident (op term1 term2)))) ;; {ident \<- term1 op term2}

here the error in Racket:

#lang reader "racket/SRFI-105.rkt"  
(require "../Scheme-PLUS-for-Racket/main/Scheme-PLUS-for-Racket/Scheme+.rkt")

Welcome to DrRacket, version 8.7 [cs].  
Language: reader "racket/SRFI-105.rkt", with debugging; memory limit: 8192 MB.

> (define i 3)  
> {i \<- i + 1}  
> \<-: bad syntax in: \<-

Regards,  
Damien

---

<div class="post-metadata">

**Author:** ![damien\_mattei](https://yyz2.discourse-cdn.com/free1/user_avatar/racket.discourse.group/damien_mattei/32/2706_2.png) [@damien\_mattei](https://racket.discourse.group/u/damien_mattei)\
**Post date:** [April 14, 2023, 7:31pm UTC](https://racket.discourse.group/t/macro-problem-i-do-not-understand/1864/2 "2023-04-14T19:31:26Z")

</div>

here is an easy to test Racket compatible version of the problem (no need of SRFI-105) ,this proove it is not related to SRFI-105 implementation:

```scheme
#lang racket
(define-syntax $nfx$
  (syntax-rules ()

    ((_ ident opspecial term1 op term2) (cond ((or (equal? (quote opspecial) (quote <-)) (equal? (quote opspecial) (quote ←)))
					       (begin
						 (display "$nfx$") (newline)
						 (opspecial ident (op term1 term2)))) ;; {ident <- term1 op term2}
					      
					      ((or (equal? (quote op) (quote ->)) (equal? (quote op) (quote →))) (op term2 (opspecial ident term1))) ;; Warning: argument names of macro do not reprensent the values contained in this case
					      
					      ;;(else (! (quote ident) (quote opspecial) (quote term1) (quote op) (quote term2)))))
					      (else (! ident (quote opspecial) term1 op term2))))))

(define-syntax <-
  
  (syntax-rules ()

    ((_ var expr)
     
     (begin
       ;;(display "<- : variable set!") (newline)
       (set! var expr)
       var))))

```

this works:

```scheme
Welcome to DrRacket, version 8.7 [cs].
Language: racket, with debugging; memory limit: 8192 MB.
> 1
1
> (define i 3)
> ($nfx$ i <- i + 1)
$nfx$
4

```

if i do not quote opspecial in the else clause which is not used! it give an error:

```scheme
#lang racket
(define-syntax $nfx$
  (syntax-rules ()

    ((_ ident opspecial term1 op term2) (cond ((or (equal? (quote opspecial) (quote <-)) (equal? (quote opspecial) (quote ←)))
					       (begin
						 (display "$nfx$") (newline)
						 (opspecial ident (op term1 term2)))) ;; {ident <- term1 op term2}
					      
					      ((or (equal? (quote op) (quote ->)) (equal? (quote op) (quote →))) (op term2 (opspecial ident term1))) ;; Warning: argument names of macro do not reprensent the values contained in this case
					      
					      ;;(else (! (quote ident) (quote opspecial) (quote term1) (quote op) (quote term2)))))
					      (else (! ident opspecial term1 op term2))))))

(define-syntax <-
  
  (syntax-rules ()

    ((_ var expr)
     
     (begin
       ;;(display "<- : variable set!") (newline)
       (set! var expr)
       var))))

```

error:

```scheme
Welcome to DrRacket, version 8.7 [cs].
Language: racket, with debugging; memory limit: 8192 MB.
> (define i 3)
> ($nfx$ i <- i + 1)
<-: bad syntax in: <-
> 

```

---

<div class="post-metadata">

**Author:** ![damien\_mattei](https://yyz2.discourse-cdn.com/free1/user_avatar/racket.discourse.group/damien_mattei/32/2706_2.png) [@damien\_mattei](https://racket.discourse.group/u/damien_mattei)\
**Post date:** [April 15, 2023, 11:36am UTC](https://racket.discourse.group/t/macro-problem-i-do-not-understand/1864/3 "2023-04-15T11:36:09Z")

</div>

just understood my error:

quote is a macro, so it stop the evaluation of the other macro \<- and \<- is expanded in the else even if this branch will not be executed and the error arise.