Last week I have been playing a lot with specs and types and as homework I created Tyyppi library that allows dealing with types in runtime. It’s currently in a kindergarten, but it already might provide some goodness, including but not limited to:

• runtime check if this term is of that type with Tyyppi.of?/2, e. g. Tyyppi.of?(GenServer.on_start(), :ok) (it’s apparently not )
• apply the function providing its spec so that it returns {:ok, result} if and only if all the arguments and the result meet the specs, and {:error, {:arity | :args | :result, value}} otherwise
• create a typed struct with generated complete detailed t() :: %...{...} type definition, type checks on upserts, and a possibility to cast values on upserts to the expected type (TBD).

I am aware of great TypeCheck library by @Qqwy but my main goal was to stay as close to standard specs as possible. Tyyppi does by no means compete with TypeCheck.