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
– Elias Mårtenson, Commented Oct 31, 2012 at 8:34

Community · Accepted Answer · 2017-05-23 10:25:10Z

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, 2017 at 10:25

CommunityBot

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answered Oct 30, 2012 at 9:55

finnw

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1 Comment

George Ogden Over a year ago

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.

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