The hs-di package

[Tags:bsd3, library, program, test]

Dependency Injection library for Haskell to allow powerful unit testing and mocking (compile-time type-checked)

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Versions 0.2.0, 0.2.1, 0.2.2, 0.3.0
Dependencies base (>=4.7 && <5), compose-ltr, containers, haskell-src-meta, template-haskell, time [details]
License BSD3
Copyright 2016 Milan Nagy
Author Milan Nagy
Stability Unknown
Category Testing, Control, Development, Dependency Injection, Template Haskell
Home page
Source repository head: git clone
Uploaded Mon Sep 26 00:18:04 UTC 2016 by Wizek
Distributions NixOS:0.3.0
Downloads 98 total (4 in the last 30 days)
0 []
Status Docs uploaded by user [build log]
Last success reported on 2016-09-26 [all 1 reports]




Maintainer's Corner

For package maintainers and hackage trustees

Readme for hs-di

Readme for hs-di-0.3.0

Haskell Dependency Injection Hackage

A promising Dependency Injection system for Haskell.


The main motivation behind this project is to make it very easy to mock dependencies of functions for unit testing, even if they are nested many levels deep.

Another motivation of mine was to find a technique that works entirely at compile time, having the following benefits:

  • compile-time type checking of all dependencies and whether they fit together
  • no run-time performance penalty
  • no run-time Dependency Injection related errors


A motivating example:

-- Lib.hs
{-# language TemplateHaskell #-}

module Lib where

import DI

noun = "World"

sentence noun = "Hello " ++ noun

statement sentence = sentence ++ "!"
-- Spec.hs
{-# language TemplateHaskell #-}

import DI
import Lib

nounMock = "Dear Reader" 

main = do
  $(assemble statementD) `shouldBe` "Hello World!"
  $(assemble $ override "noun" "nounMock" $ statementD) `shouldBe` "Hello Dear Reader!"

-- assertion function
shouldBe = shouldBeF show
shouldBeF f actual expected | actual == expected = putStrLn $ "OK " ++ f actual
                            | otherwise          = error $ "FAIL " ++ f actual ++ " /= " ++ f expected

Which when executed should output:

OK "Hello World!"
OK "Hello Dear Reader!"

Observe: In the second assertion, noun is being overridden while we are testing statement. noun is not an immediate dependency of statement but a dependency at 2 levels deep.


In this project I am trying to emulate the manual assembly of deeply nested and injected dependencies with the help of TemplateHaskell and compile-time dependency graphs as configuration.

To go into more details, this is what happens behind the scenes in the above example:

Lib.hs:8:1-3: Splicing declarations
    nounD = Dep "noun" []
    nounT = (noun)
    nounA = noun
    nounI = noun
Lib.hs:11:1-3: Splicing declarations
    sentenceD = Dep "sentence" [nounD]
    sentenceT = (sentence, nounT)
    sentenceA = sentence nounA
    sentenceI = sentence
Lib.hs:14:1-3: Splicing declarations
    statementD = Dep "statement" [sentenceD]
    statementT = (statement, sentenceT)
    statementA = statement sentenceA
    statementI = statement
Spec.hs:7:1-3: Splicing declarations
    nounMockD = Dep "nounMock" []
    nounMockT = (nounMock)
    nounMockA = nounMock
    nounMockI = nounMock
Spec.hs:11:5-23: Splicing expression
    assemble statementD
    let (statement, (sentence, noun)) = statementT
    in statement (sentence noun)
Spec.hs:12:5-54: Splicing expression
    assemble $ override "noun" "nounMock" $ statementD
    let (statement, (sentence, _)) = statementT
    in statement (sentence nounMock)

A couple things to note:

  • You may be wondering what the suffix letters mean in the declarations.
    You don't have to concern yourself with them, it's part of the internal hidden API of the DI framework by design.
    (If you are curious however, they stand for "Dependency definitions/Deps", "Tuple", "Assembled", and "Injectable", respectively.)
  • As you can see, at the end of the day, all this machinery achieves pretty much the same what a developer would do by hand: statement (sentence noun)
    The beauty, however, is that this doesn't have to be done by hand, as it would become immensly tideous and time-consuming as soon as we start to handle more than a couple dependencies.
  • Mocking is equally elegant:
    let (statement, (sentence, _)) = statementT in statement (sentence nounMock)
    (translated from $(assemble $ override "noun" "nounMock" $ statementD))

See more advanced example below.

To try

To execute the above example:

git clone
cd hs-di/examples/simple
stack test

You may also experiment with modifying the files in hs-di/examples/simple then re-running stack test to get an intuitive understanding of how this library works.

More advanced example

While the following code may not be the most elegant or useful, it at least shows the power of dependency injection when it comes to mocking and testing IO code that deals with putStrLn and getCurrentTime in a fully deterministic way.

makeTimer putStrLn getCurrentTime = liftIO $ do
  prevTime <- newIORef Nothing
  return $ liftIO $ do
    pTime <- readIORef prevTime
    time <- getCurrentTime
    writeIORef prevTime $ Just time
    case pTime of
      Nothing -> putStrLn $ show time
      Just a  -> putStrLn $ show time ++ ", diff: " ++ (show $ diffUTCTime time a)


timer <- $(makeTimerD
    $> override "putStrLn" "putStrLnMock"
    $> override "getCurrentTime" "getCurrentTimeMock"
    $> assemble

readMockConsole `shouldReturn` []

writeIORef cTime $ parseTime "2016-01-01 14:00:00"
readMockConsole `shouldReturn` ["2016-01-01 14:00:00 UTC"]

writeIORef cTime $ parseTime "2016-01-01 14:00:01"
readMockConsole `shouldReturn`
  ["2016-01-01 14:00:00 UTC", "2016-01-01 14:00:01 UTC, diff: 1s"]

*excerpt from:*

Pros and cons of this approach

  • (+) Supports values to be injected
  • (+) Supports functions to be injected
  • (+2) Supports overriding of arbitrary number and depth of dependencies
  • (+2) Compile time type checking (despites strings being used, those too are checked)
  • (+) Supports type variables
  • (+) Theoretically also supports surgically only overriding some subsets of dependencies
  • (+) Emulates how a human would do DI by hand, and does the hard work automatically
  • (+) Some module support
    • (-.5) The module support is not yet fully perfect
    • (-.5) Due to limitations of Template Haskell declaration splices, "variable not in scope" errors can pop up that are annoying. Although it is in theory possible to work around these, and it is planned for a later release.
  • (?) How is performance impacted? Does GHC notice f (g x) (g x)?


This package was initially inspired by the Dependency Injection framework of AngularJS (1.x).
Additional inspiration came when I was looking for ways to make DI work in a statically typed language at compile time, and found out about Dagger (Java).

Todo checklist

  • [x] v0.2+ make multiple arguments work
  • [x] v0.2+ Simplify Deps
  • [x] v0.2+ reorder arguments of override
  • [x] v0.2+ try with some real-life code
  • [x] v0.2+ Write quasi quoter or TH splicer that writes the Deps definitions too
  • [x] v0.2+ look for a way to have full module support (without having to explicitly re-export and risk name-clashes)
  • [x] v0.3+ Support function headers that are not immediately below
    • [ ] Consider using haskell-source-meta to extract parameter info
  • [x] v0.3+ work around "variable not in scope" error by collecting all declarations in a splice at the end of the file
  • [x] v0.3+ Allow single dependency more than once
  • [ ] have GHC support Dec TH splices in let bindings: Which could make overriding dependencies with mocks more pleasant
  • [ ] have GHC lift stage restriction

Experimental Features

"Inject Gradual": Gradually introduce DI

-- Lib.hs
{-# language TemplateHaskell #-}

module Lib where

import DI

nounI = "World"

sentence :: String
sentenceI noun = "Hello " ++ noun

statementI sentence = sentence ++ "!"

legacyStatement = sentence ++ "..."

The injG top level Q [Dec] splice requires the dependency name to end with the suffix I, and defines an injected (assembled) value without the suffix to be used in legacy code not yet part of dependency injection. E.g. nounI --> noun. This allows for more gradual transition of a codebase into using DI, since declarations can be updated one at a time while allowing the program to remain able to be compiled and identical in terms of execution and behaviour.

Lib.hs:11:1-3: Splicing declarations
    sentenceD = Dep "sentence" [nounD]
    sentenceT = (sentenceI, nounT)
    sentenceA = sentenceI nounA
    sentence = sentenceA

Inject All

-- Lib.hs
{-# language TemplateHaskell #-}

module Lib where

import DI


sentenceI noun = "Hello " ++ noun
nounI = "World"
statementI sentence = sentence ++ "!"

This allows us to overcome multiple limitations of TemplateHaskell having to do with scoping, and avoid errors such as Lib.hs:10:1: Not in scope: ‘noun’. It also allows us to define our declarations in arbitrary order.

Lib.hs:8:1-7: Splicing declarations
    sentenceD = Dep "sentence" Original Pure [nounD] :: Deps
    sentenceT = (sentenceI, nounT)
    sentenceA = sentenceI noun
    sentence = sentenceA
    nounD = Dep "noun" Original Pure [] :: Deps
    nounT = (nounI)
    nounA = nounI
    noun = nounA
    statementD = Dep "statement" Original Pure [sentenceD] :: Deps
    statementT = (statementI, sentenceT)
    statementA = statementI sentence
    statement = statementA

Some further reading on the subject:

Override inline

This allows us to define short and ad-hoc mocks inline

$(assemble $ override "noun" "\"there\"" $ statementD) `shouldBe` "Hello there!"

Alternatively, if one uses a HereDoc such as interpolatedstring-perl6 dealing with quotation marks can be simpler:

$(assemble $ override "noun" [qc|"there"|] $ statementD) `shouldBe` "Hello there!"