To disallow invalid input it is common to define (new)types with hidden data
constructors. Forcing the user to go through a smart-constructor that enforces
invariants and returns Maybe ResultType, preventing the construction of data
with invalid values.
However, it is also common to want to include literal values of such types
in source text. Things of textual literals for HTML, HTTP, etc. In such cases
smart-constructors force us to handle potential conversion failures at runtime,
or abusing functions like fromJust to break away all the safety
smart-constructors provide. All this despite the fact that we can statically
know at compile time that the conversion will always succeed or always fails.
This package provides a typeclasses for using TH to validate the correctness of
provided literals at compile. This lets you define, e.g., newtype Even = Even Integer and write:
x :: Even
x = $$(valid 38)
This will check, at compile time, that the provided Integer is, in fact, even
and unwrap it from Maybe, avoiding the runtime check.