Uni-directional coercions, or representational subtyping relations.
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This package provides Attenuations, which act as uni-directional coercions, or
representational subtyping relations.
Like Coercions, these can be used directly to coerce between their type
parameters at zero runtime cost, or lifted to operate over "larger" types.
Unlike Coercions, these can be used only in one direction, from a "weaker"
type to a "stronger" type. Accordingly, they have additional restrictions on
lifting through type constructors: they must respect the variance (covariance or
contravariance) of the type being lifted. Lifting an Attenuation covariantly
lets you coerce from, e.g., [Fin n] -> [Int], but not vice versa; while
lifting contravariantly lets you coerce from e.g. Op Bool Int -> Op Bool (Fin n), but not vice versa.