program: Programs with Environments and Managed Resources

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Dependencies base (>=4.13 && <5) [details]
License BSD-3-Clause
Copyright 2021 Michael Szvetits
Author Michael Szvetits
Revised Revision 1 made by MichaelSzvetits at 2021-12-03T12:57:54Z
Category Control
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Uploaded by MichaelSzvetits at 2021-12-02T16:14:35Z
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Last success reported on 2021-12-02 [all 1 reports]

Readme for program-

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program is a library for writing programs with environments and managed resources, written in Haskell. It aims to be simple, has minimal dependencies and combines features of various existing approaches for threading an environment through an application (e.g., RIO, ReaderT, Handle Pattern, record-of-functions) and for managing resources without nested bracket-functions (e.g., managed). The library and its documentation can be found on Hackage.


The following simple example copies count characters from one file to another, where count is demanded from the environment e by the function copy (via pull) and supplied by main. Note that we do not need to close file handles, because resources are managed automatically.

{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE TypeOperators    #-}
module Example where

-- base
import Control.Monad
import Control.Monad.IO.Class
import System.IO

-- program
import Control.Program (Program, Has, manage, pull, runProgram)

copy :: e `Has` Int => FilePath -> FilePath -> Program e ()
copy from to = do
  fromHandle <- manage (withFile from ReadMode)
  toHandle   <- manage (withFile to WriteMode)
  count      <- pull
  liftIO . replicateM_ count $
    hGetChar fromHandle >>= hPutChar toHandle

main :: IO ()
main = do
    ( 10 :: Int )
    ( copy "/tmp/source" "/tmp/target" )

In larger applications, the environment would contain many more complex values (all demanded by Has) and can also be used to manage mutable state. See the documentation on Hackage for more details.


  • Easy to understand (e.g., no unlifting, no type-level wizardry, hardly any language extensions).
  • No need for extra dependencies. All we need is base.
  • Mocking is easy by supplying different environments via runProgram.
  • No fight with the type inference (i.e., down-to-earth types, hardly any typeclasses).
  • The environment can easily simulate beloved effects like Reader and State.
  • Easy integration of other effects by putting records-of-functions into the environment.
  • Clear error message because involved types are not overly generic.