Use this type in a function embedded in a Command in order to require a flag. The type signature for this lists the flags that we want to match against, the associated help text, and the output type we want the flag to be cast to.

Flags of Maybe or Bool types have special handling: if the flag doesnt exist for either of these types, we'll be handed back a False/Nothing rather than get an error, else if the flag does exist we'll get back a True/Just val. Bool flags are expected to be provided an empty string as the value; if you care about the value but want it to be optional, use Maybe.

Use this type in a function embedded in a Command in order to require a value. The type signature for this contains the associated help text for the command, and the type we expect the value to be cast to.

Typeclass used by Flag and Value to convert the provided string to the desired haskell type. Anything that satisfies Read will satisfy this, but we can override the Read behaviour as we see fit on a per type basis by explicitly implementing this.