Il giorno dom 25 nov 2018 alle ore 14:21 swedebugia <
address@hidden> ha scritto:
Hi
I am still a novice in guile so I humbly ask for help with this error
trying to get the blacklisting to work:
address@hidden ~$ ~/guix-tree/pre-inst-env guix import npm leaflet
ice-9/boot-9.scm:222:17: In procedure map1:
Syntax error:
/home/sdb/guix-tree/guix/import/npm.scm:304:2: source _expression_ failed
to match any pattern in form (if (member (cut guix-name "node-" <>)
blacklist))
The relevant code is in npm.scm as detailed in the error above.
The files are attached.
Thanks in advance!
There are 2 observations I can give
The first one is that member" returns a boolean value (#t or #f) and your function can return just that, you don't need the if
But even if you needed the if, you have to pass it at least 2 forms: a test and another _expression_ whose value will be returned if the tests succeeds (that is, it evaluates to #t)
In your code you are passing the if form only one _expression_.
The only _expression_ you are passing to the if is
(member (cut guix-name "node-" <>) blacklist)
if this subexpression evaluates to #t, what is the if supposed to do ?
You could have written
(define (blacklisted? guix-name)
"Check if guix-name is blacklisted. RETURN #t if yes, else #f."
(if (member (cut guix-name "node-" <>) blacklist) #t #f))
in this exaple, if the subexression evaluates to #t, the if form can return #t, which is
but as you can see (this is my second observation), this formulation is a bit redundant
It can be rewritten without the if at all, lie this
(define (blacklisted? guix-name)
"Check if guix-name is blacklisted. RETURN #t if yes, else #f."
(member (cut guix-name "node-" <>) blacklist)))
In this formulation, if the subexpression
(member (cut guix-name "node-" <>) blacklist))
returns #t, your function will return #t, because your function returns the value off its last subform
if the subexpression returns #f, of course your function will return #f
That's supposedly what you wanted to achieve, isn't it ?
You can achieve it without the if 😊
Usually when Guile laments that a source _expression_ doesn't match any pattern, that's because you messed up the parenses or because you passed the wrong number of subexpressions to a form
Please note that in this case Guile is not denouncing that the wrong number of arguments were passed to a function
Because the "if" is not a function, it's a special form (I think), and the expander tries to recognize a known "pattern" in it