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In Vimscript, if you use a function as an R-value (right hand side value) of an assignment statement, then the function simply evaluates. For example:

let count_pattern += str2nr( strpart( execute( command ), 1 ) )

However, if a function is used not as an R-value of an assignment statement, then a "call" keyword must be used:

call substitute( current_line, '\d\+', '\=add( listValues, submatch(0) )', 'g')

What is the necessity for the use of the "call" keyword? In most languages one can simply use a function by itself as a statement, but apparently in Vimscript you cannot.

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Tyler Durden
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  • I have been scripting vim for over a decade and have nearly 4k lines in my vimrc. I use vim very heavily. Only today did I actually learn the pattern behind when I'm supposed to use call. Thank you so much for clarifying it in your question. – Steven Lu Nov 09 '20 at 02:13
  • @StevenLu Uh, ok. Are you going to upvote the question then? – Tyler Durden Nov 09 '20 at 02:45
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    Ask and you shall receive. – Steven Lu Nov 09 '20 at 05:02

2 Answers2

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IMO, we have to consider that vi and ed had ex-commands, and that vim introduces functions.

When vim interprets a line, it needs a way to distinguish :substitute from substitute(), or :MyUserThing (which is a command) from MyUserThing() (which is a function).

That's why :call is required. As :let, this is an ex-command. The difference between these two is that the result will be ignored as we don't need it, but this is anecdotal. What's important is to be able to distinguish ex-commands from function calls.

Luc Hermitte
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  • Ahh. So, in the question of OP, the "functions" in the R-value is not a function call in the first place, rather they're ex commands? Or is it that these are actually functions but when used in R-value, they act as ex commands? – 3N4N Mar 13 '19 at 17:15
  • @klaus The OP was wondering why he couldn't write :substitute( current_line, '\d\+', '\=add( listValues, submatch(0) )', 'g') without the :call. The reason is likely that Vim has no way to know that the end-user wants to call a function and not a command with (whatever) as command parameter. :call is required to remove the ambiguity. – Luc Hermitte Mar 13 '19 at 17:20
  • But couldn't that be made unambiguous by checking out that substitute has () after it? And my query was in let count_pattern += str2nr( strpart( execute( command ), 1 ) ), is execute(command) a function call or an ex command? – 3N4N Mar 13 '19 at 17:38
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    @klaus No because (whatever) could be a perfect parameter for a command. – Luc Hermitte Mar 13 '19 at 17:43
  • @klaus, the ex-command is :let here. What is after the equal sign is an expression. – Luc Hermitte Mar 13 '19 at 17:44
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Hypothesis: When you're using a function as an R-value, or right hand side value, of an assignment statement, you're trying to assign the output of the R-value to the L-value variable.

Why call is needed when calling a function: From the documentation (see :h call) it is evident that call will discard the return value of a function.

                                        *:cal* *:call* *E107* *E117*
:[range]cal[l] {name}([arguments])
        Call a function.  The name of the function and its arguments
        are as specified with |:function|.  Up to 20 arguments can be
        used.  The returned value is discarded.

So, since when you're using functions as R-value you don't want to discard the output of the function, rather want to assign it to the L-value, you don't use call.

Moreover, you don't always need call to call a function. You can pass the function to another function for example,

:function GetMeow()
:  return "Meow String!"
:endfunction

:GetMeow()       " ERROR!


:call GetMeow()  " no output


:echom GetMeow() " shows "Meow String!"

Edit

Many languages have the notion of "statements" and "expressions" and programs are generally sequences of statements. In C, GetMeow() may be an expression but GetMeow(); (with a semicolon) is a statement. In vim, GetMeow() is an expression but call GetMeow() is a statement, which are referred to as ex commands. It's really not as strange as the question implies.

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3N4N
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  • GetMeow() example is from Learn Vimscript The Hard Way by the way. – 3N4N Mar 09 '19 at 06:08
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    vimscript is a sequence of (ex) commands, not expressions, and only some of these commands interpret expressions (call, echom, let, etc). I see new users frequently confused by why say, :set option=g:value or :buffer g:buf won't work. some commands like execute and echo are even capable of interpreting multiple expressions: e.g., echo 1 + 2 3 + 4 is well formed – Mass Mar 09 '19 at 15:34
  • I don't understand. Did I leave something out of my answer? Or is there a mistake? You can edit the answer if you want. – 3N4N Mar 09 '19 at 16:08
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    I don't really see how the quoted documentation for :call explains why :GetMeow() is an error. From my understanding, it's the latter that the question is really about. @Mass's comment seems to touch on the reasoning, but could do with some fleshing out before becoming a full answer (or possibly integrated into this one). – 8bittree Mar 12 '19 at 19:22
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    @klaus, Although your answer is factual, I felt like it did not address a critical distinction. Many languages have the notion of "statements" and "expressions" and programs are generally sequences of statements. In C, GetMeow() may be an expression but GetMeow(); (with a semicolon) is a statement. In vim, GetMeow() is an expression but call GetMeow() is a statement, which are referred to as ex commands. It's really not as strange as the question implies. – Mass Mar 12 '19 at 19:37
  • @Mass, I had no idea, because I have zero knowledge of vimscript as a language. Anyway, I added your comment to my answer. The thing is I didn't even think about that line of thought because vimscript always felt kinda like python, and python AFAIK has little distinction between statements and expressions. And you should edit the answer, it will become more clear then to understand. Thanks. – 3N4N Mar 13 '19 at 02:34
  • @8bittree, I thought in vimscripts, functions needed to be called with call keyword because otherwise the return value would not be used. Kind of like how C compilers give warning when a function returns something but you don't store it in a variable. Anyway, I edited my answer. Please, check if everything is alright. – 3N4N Mar 13 '19 at 02:37
  • how to keep the return value? – NeoZoom.lua Feb 03 '22 at 12:42