if var
if var
If a variable is the condition of an if
, inside the then
branch the variable will be considered as not having the Nil
type:
a = some_condition ? nil : 3 # a is Int32 or Nil if a # Since the only way to get here is if a is truthy, # a can't be nil. So here a is Int32. a.abs end
This also applies when a variable is assigned in an if
's condition:
if a = some_expression # here a is not nil end
This logic also applies if there are ands (&&
) in the condition:
if a && b # here both a and b are guaranteed not to be Nil end
Here, the right-hand side of the &&
expression is also guaranteed to have a
as not Nil
.
Of course, reassigning a variable inside the then
branch makes that variable have a new type based on the expression assigned.
The above logic doesn’t work with instance variables or class variables:
if @a # here @a can be nil end
This is because any method call could potentially affect that instance variable, rendering it nil
. Another reason is that another thread could change that instance variable after checking the condition.
To do something with @a
only when it is not nil
you have two options:
# First option: assign it to a variable if a = @a # here a can't be nil end # Second option: use `Object#try` found in the standard library @a.try do |a| # here a can't be nil end
That logic also doesn't work with proc and method calls, including getters and properties, because nilable (or, more generally, union-typed) procs and methods aren't guaranteed to return the same more-specific type on two successive calls.
if method # first call to a method that can return Int32 or Nil # here we know that the first call did not return Nil method # second call can still return Int32 or Nil end
The techniques described above for instance variables will also work for proc and method calls.
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https://crystal-lang.org/docs/syntax_and_semantics/if_var.html