module T

def self.let(value, type, checked: true)

doesn't match the type.
If `checked` is true, raises an exception at runtime if the value

Compared to `T.cast`, `T.let` is _checked_ by static system.

seconds = T.let(0.0, Float)

like:
Tells the typechecker to declare a variable of type `type`. Use 
def self.let(value, type, checked: true)
  return value unless checked
  Private::Casts.cast(value, type, "T.let")
end