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.
