Dependent integer literals.
This plugin implements pseudo-dependently-typed integer literals.
This means the value of an integer literal is available at the type level, and can be used for things like validation or type inference. For example, the SInt instance refines unknown type-level Nat variables to the particular literal value, and the Fin instance emits compile-time errors for out-of-range values. So, given mkVec :: SInt n -> (Fin n -> a) -> Vec n a, mkVec 4 will have type (Fin 4 -> a) -> Vec 4 a, and trying to type-check 4 :: Fin 4 will report a type error saying that 4 is out of range.
Patterns get a similar treatment, too, so case (x :: SInt n) of { 1 -> Just Refl; _ -> Nothing } :: Maybe (n :~: 1) can type-check: the act of matching against 1 proved that the type-level n was equal to 1, so Refl can type-check as n :~: 1.
To use this, enable the extensions DataKinds, FlexibleContexts, ViewPatterns, and TypeApplications, add dependent-literals and dependent-literals-plugin to your package's dependencies, and add -fplugin=DependentLiterals.Plugin to ghc-options to enable the plugin globally, or use OPTIONS_GHC pragmas to enable the plugin on a per-module basis. "Normal" integral types should still behave as normal, but literals and numeric patterns will become supported for SInt and Fin, and will provide additional information to the type checker automatically.