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/Crystal 0.31

if var.nil?

If an if's condition is var.nil? then the type of var in the then branch is known by the compiler to be Nil, and to be known as non-Nil in the else branch:

a = some_condition ? nil : 3
if a.nil?
  # here a is Nil
else
  # here a is Int32
end

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https://crystal-lang.org/docs/syntax_and_semantics/if_var_nil.html