Re: Bug in my WIP-npm-importer with blacklist

[Catonano <catonano@gmail.com>]

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Il giorno dom 25 nov 2018 alle ore 14:21 swedebugia <swedebugia@riseup.net>
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:
>
> sdb@komputilo ~$ ~/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

It fails and it gives up

Hope this helps

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