# validated-literals: Compile-time checking for partial smart-constructors

[ bsd3, data, library ] [ Propose Tags ]

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. [Skip to Readme] ## Modules [Index] [Quick Jump] ## Downloads Note: This package has metadata revisions in the cabal description newer than included in the tarball. To unpack the package including the revisions, use 'cabal get'. #### Maintainer's Corner Package maintainers For package maintainers and hackage trustees Candidates • No Candidates Versions [RSS] 0.1.0, 0.2.0, 0.2.0.1, 0.3.0, 0.3.1 (info) CHANGELOG.md base (>=4.9 && <4.18), template-haskell (>=2.11 && <2.20), th-compat (>=0.1.3 && <0.2) [details] BSD-3-Clause Copyright © 2015-2021 Merijn Verstraaten Merijn Verstraaten Merijn Verstraaten Revision 1 made by MerijnVerstraaten at 2022-08-24T11:53:50Z Data https://github.com/merijn/validated-literals https://github.com/merijn/validated-literals/issues head: git clone git+ssh://github.com:merijn/validated-literals by MerijnVerstraaten at 2021-12-02T13:09:26Z NixOS:0.3.1 1 direct, 0 indirect [details] 2137 total (20 in the last 30 days) (no votes yet) [estimated by Bayesian average] λ λ λ Docs uploaded by userBuild status unknown ## Readme for validated-literals-0.3.1 [back to package description] # validated-literals: Compile-time checking for partial smart-constructors 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.