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Editorial Team
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Asked: 2026-06-13T12:22:05+00:00

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

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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.

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1 Answer

  1. Editorial Team

    You want

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

    instead of

    (when (func x) (list x))

    More information about function vs. variable namespace:

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