reflection: Reifies arbitrary terms into types that can be reflected back into terms

[ bsd3, data, dependent-types, library, reflection ] [ Propose Tags ]

This package addresses the configuration problem which is propogating configurations that are available at run-time, allowing multible configurations to coexist without resorting to mutable global variables or System.IO.Unsafe.unsafePerformIO.

An example is modular arithmetic where the modulus itself can be supplied at run-time:

foo :: Modular s => Modulus s
foo = 1000 * 1000 * 5 + 2000
>>> withModulus 1280 foo

given the following setup:

{-# LANGUAGE ScopedTypeVariables, RankNTypes, ConstraintKinds, FlexibleContexts, UndecidableInstances 125;

import Data.Proxy      (Proxy(Proxy))
import Data.Reflection (Reifies, reflect, reify)

and definitions:

data Modulus s = M { getModulus :: Integer }
type Modular s = Data.Reflection.Reifies s Integer

normalize :: forall s. Modular s => Integer -> Modulus s
normalize n = M (mod n modulus) where
   modulus = Data.Reflection.reflect (Data.Proxy.Proxy :: Data.Proxy.Proxy s)

instance Modular s => Num (Modulus s) where
   M a + M b = normalize (a + b)
   M a * M b = normalize (a * b)

withModulus :: Integer -> (forall s. Modular s => Modulus s) -> Integer
withModulus m v = Data.Reflection.reify m (getModulus . asProxyOf v)
   asProxyOf :: f s -> Proxy s -> f s
   asProxyOf = const

That package is an implementation of the ideas presented in the paper "Functional Pearl: Implicit Configurations" by Oleg Kiselyov and Chung-chieh Shan (original paper). However, the API has been streamlined to improve performance.

Austin Seipp's tutorial Reflecting values to types and back provides a summary of the approach taken by this library, along with more motivating examples.

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Change log CHANGELOG.markdown
Dependencies base (>=2 && <4.12), tagged (>=0.4.4 && <1), template-haskell [details]
License BSD-3-Clause
Copyright 2009-2013 Edward A. Kmett, 2012 Elliott Hird, 2004 Oleg Kiselyov and Chung-chieh Shan
Author Edward A. Kmett, Elliott Hird, Oleg Kiselyov and Chung-chieh Shan
Maintainer Edward A. Kmett <>
Revised Revision 1 made by phadej at 2018-09-24T16:48:51Z
Category Data, Reflection, Dependent Types
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Source repo head: git clone git://
Uploaded by EdwardKmett at 2015-03-08T09:08:45Z
Distributions Arch:2.1.6, Debian:2.1.4, Fedora:2.1.6, FreeBSD:2, LTSHaskell:2.1.6, NixOS:2.1.6, Stackage:2.1.6, openSUSE:2.1.6
Downloads 201205 total (1661 in the last 30 days)
Rating 2.25 (votes: 2) [estimated by Bayesian average]
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Status Hackage Matrix CI
Docs available [build log]
Last success reported on 2015-03-08 [all 1 reports]



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Build Status

This package provides an implementation of the ideas presented in Functional Pearl: Implicit Configurations by Oleg Kiselyov and Chung-Chieh Shan. However, the API has been implemented in a much more efficient manner.

Contact Information

Contributions and bug reports are welcome!

Please feel free to contact me through github or on the #haskell IRC channel on

-Edward Kmett