[Proposal] add "when" to @type and @typep syntax

[Rich Morin]

While editing a file of @type statements, it occurred to me that it would be
convenient to be able to abridge them like @spec statements. For example:

@type params :: %{ String.t => String.t }

could be rewritten as:

@type params :: %{ s => s } when s: String.t

Of course, this should also be possible with @typep statements.

Nu?

-r

[Paul Schoenfelder]

This is mostly possible already, like so:

@type params(k, v) :: %{k => v}

@spec handle_params(params(s, s)) :: ok when s :: String.t

While Erlang doesn't support expressing guards on the generic parameters in a type definition, Elixir could just convert such definitions to the Erlang compatible form by replacing the type variables with the concrete type. In general though I tend to deal with the verbosity problem by extracting the noisy type name into its own definition that is more easily used, e.g.:

@type param_key :: String.t

@type param_val :: String.t

@type params :: %{param_key => param_val}

This has the advantage of being easier to read and easier to change in the future (the `params` type doesn't need to change if the range of types for keys/values changes).

Paul

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