Is this an empty value?

nullp expression
null? expression

Returns true if the result of the expression is an empyt value (a variable do exists, but it is not initialized; a function with variable return type or a function called dynamically did not return any value).

Example 1:

local "x
print null? :x
make "x 1
print null? :x

Output:

True
False

Example 2:

to fn :x
  ifelse :x > 0 [output 1] [stop]
end

print null? fn 1
print null? fn -1

Output:

False
True

See also:

Type predicates

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