responds_to?

The pseudo-method responds_to? determines whether a type has a method with the given name. For example:

a = 1
a.responds_to?(:abs)    #=> true
a.responds_to?(:size) #=> false

It is a pseudo-method because it only accepts a symbol literal as its argument, and is also treated specially by the compiler, as explained in if var.responds_to?(...).

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