Type-indexed maps for fun and functional dependency injection

gistfile1.txt

Type-indexed maps (TMap) are data structures that support type-safe,
constant time lookups of elements by their type. They are unordered,
allow structural typing and can be used as composable records. Unlike
HList-based implementations as in Shapeless, TMaps are implemented
using subtyping, which leads to pretty readable types and error messages.
TMap types can easily be written by hand.

TMaps are a perfect fit for monadic dependency injection. They enable
the Reader monad to be used for multiple dependencies with zero boiler
plate. The combination can be viewed as a partial implementation of
implicit parameters as a library rather than a compiler feature. Unlike
Scala's native implicit parameters, this allows complete type-inference
without having to specify them in every method signature.

TMaps can also be used to emulate named arguments, tuples and structural typing.