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I have a list whose elements are (symbol or list) and I would like to replace the inner lists by their first elements. I do this

f[list_] := Replace[ list , x_List -> First[x], 1 ]

If I test it 

f[{{a,b},c}]

I get expected result

{a,c}

But I get an error too

First::normal: "Nonatomic expression expected at position 1 in First[x].

Why do I get the error?

edit: I can alter the definition of f such that I do not get the error, but I still do not understand why I got the error in the first place.

version 1:(based on J.M. comment)

f[list_] := Replace[list, x_List :> x, 1]

version 2:

f[list_] := Replace[list, {x_, (_) ...} -> x, 1]
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    $\begingroup$ Use :> instead of ->. $\endgroup$
    – J. M.'s persistent exhaustion ♦
    May 13, 2016 at 10:16
  • $\begingroup$ You get this error message when you apply function First to element c like such First @ c since c is an atomic element there is nothing to apply First to $\endgroup$
    – e.doroskevic
    May 13, 2016 at 10:30
  • $\begingroup$ @E.Doroskevic But c should not match the pattern x_List or does it match? $\endgroup$
    – tom
    May 13, 2016 at 10:49
  • $\begingroup$ It doesn't match, yes, but you are using Rule instead of RuleDelayed. One evaluates its right-hand side at once; the other doesn't. $\endgroup$
    – J. M.'s persistent exhaustion ♦
    May 13, 2016 at 11:01
  • $\begingroup$ @tom you can see more information about it here in the documentation: reference.wolfram.com/language/ref/Rule.html check Background & Context. 2nd paragraph $\endgroup$
    – e.doroskevic
    May 13, 2016 at 11:03

1 Answer 1

Reset to default
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f2[l_] := # & @@@ l;
f2[{{a, b}, c}]

{a, c}

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