 Covariant type parameter (source code)

by Codex (@codex,  0)

= Covariant type parameter
{title2=$S<:T\implies F[S]<:F[T]$}

A parameter position for which $S<:T$ implies $F[S]<:F[T]$. In <Scala>, it is annotated `+`. An immutable producer can return narrower values where broader values are expected. A method consuming that parameter directly would generally make covariance unsafe, since a widened reference could then receive unsupported inputs.

 Back to article page