lisp: lambda scope

Question

I want to implement filter function that would filter a list based on a condition

(defun filter (func xs)                                                          
  (mapcan                                                                        
    (lambda (x)                                                                  
      (when (func x) (list x))) xs ))

but I get an error:

*** - EVAL: undefined function FUNC

I thought that lambda should see func. How to pass func to lambda correctly?

I use CLISP.

You might also want to have a look at the function `REMOVE-IF-NOT` which does exactly this. — Elias Mårtenson, Oct 31 '12 at 08:34

score 6 · Accepted Answer · edited May 23 '17 at 10:25

6

You want

(when (funcall func x) (list x))

instead of

(when (func x) (list x))

More information about function vs. variable namespace:

edited May 23 '17 at 10:25

Community

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answered Oct 30 '12 at 09:55

finnw

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Just mentioning that `(func x)` tries to evaluate a function called `func` whereas `(funcall func x)` call a function called `func` with `x` as its argument. – George Ogden Aug 18 '22 at 11:17

lisp: lambda scope

1 Answers1