:mini:`nil` is used throughout Minilang to denote the absence of any other value. This includes the initial values of variables or object fields before assignment, indexing a :mini:`map` or :mini:`list` with a missing key or index and the result of binary comparisons which are logically false.
var X
X :> nil
let L := [1, 2, 3]
L[4] :> nil
let M := {"A" is 1, "B" is 2}
M["C"] :> nil
2 < 1 :> nil
As a result, it is often required to check the result of an expression for :mini:`nil` before using the result in a function call.
let M := {"A" is [1, 2], "B" is [3, 4]}
M["A"][1] :> 1
M["C"][1] :> MethodError: no method found for [](nil, int32)
if let T := M["A"] then T[1] end :> 1
if let T := M["C"] then T[1] end :> nil
Minilang provides a shorthand for this type of check using guarded arguments. A simple guarded argument consists of an empty pair of braces :mini:`{}` following an expression in a function call.
.. parser-rule-diagram:: expression '{' '}'
If the expression in a simple guarded argument evaluates to :mini:`nil` then the innermost function call containing the guarded argument evaluates immediately to :mini:`nil` without invoking the called function.
let M := {"A" is [1, 2], "B" is [3, 4]}
M["A"]{}[1] :> 1
M["C"]{}[1] :> nil
Other than simply checking for :mini:`nil`, guarded arguments can also check their preceeding expression for arbitrarily complex conditions by including another expression inside the braces. Within the braces, the keyword :mini:`it` refers to the value of the preceeding expression.
.. parser-rule-diagram:: expression '{' expression '}'
for I in 1 .. 10 do
print("I = ", I{2 | it}, "\n")
end
I = 2
I = 4
I = 6
I = 8
I = 10