strict-base-types: Strict variants of the types provided in base.

[ bsd3, data, library ] [ Propose Tags ]

It is common knowledge that lazy datastructures can lead to space-leaks. This problem is particularly prominent, when using lazy datastructures to store the state of a long-running application in memory. The easiest solution to this problem is to use fully strict types to store such state values. By "fully strict types" we mean types for whose values it holds that, if they are in weak-head normal form, then they are also in normal form. Intuitively, this means that values of fully strict types cannot contain unevaluated thunks.

To define a fully strict datatype, one typically uses the following recipe.

  1. Make all fields of every constructor strict; i.e., add a bang to all fields.

  2. Use only strict types for the fields of the constructors.

The second requirement is problematic as it rules out the use of the standard Haskell Maybe, Either, and pair types. This library solves this problem by providing strict variants of these types and their corresponding standard support functions and type-class instances.

Note that this library does currently not provide fully strict lists. They can be added if they are really required. However, in many cases one probably wants to use unboxed or strict boxed vectors from the vector library ( instead of strict lists. Moreover, instead of Strings one probably wants to use strict Text values from the text library (

This library comes with batteries included; i.e., missing instances for type-classes from the deepseq, binary, aeson, QuickCheck, and lens packages are included. Of particluar interest is the Strict type-class provided by the lens library ( It is used in the following example to simplify the modification of strict fields.

(-# LANGUAGE TemplateHaskell #-)   -- replace with curly braces,
(-# LANGUAGE OverloadedStrings #-) -- the Haddock prologues are a P.I.T.A!

import           Control.Lens ( (.=), Strict(strict), from, Iso', makeLenses)
import           Control.Monad.State.Strict (State)
import qualified Data.Map                   as M
import qualified Data.Maybe.Strict          as S
import qualified Data.Text                  as T

-- | An example of a state record as it could be used in a (very minimal)
-- role-playing game.
data GameState = GameState
    ( _gsCooldown :: !(S.Maybe Int)
    , _gsHealth   :: !Int
    )  -- replace with curly braces, *grmbl*

makeLenses ''GameState

-- The isomorphism, which converts a strict field to its lazy variant
lazy :: Strict lazy strict => Iso' strict lazy
lazy = from strict

type Game = State GameState

cast :: T.Text -> Game ()
cast spell =
    gsCooldown.lazy .= M.lookup spell spellDuration
    -- ... implement remainder of spell-casting ...
    spellDuration = M.fromList [("fireball", 5)]

See for a gentle introduction to lenses and state manipulation.

Note that this package uses the types provided by the strict package (, but organizes them a bit differently. More precisely, the strict-base-types package

Versions [faq] 0.1, 0.2, 0.2.1, 0.2.2, 0.2.3, 0.3.0, 0.4.0, 0.5.0, 0.6.0, 0.6.1
Change log CHANGES
Dependencies aeson (>=0.6), base (>=4.5 && <5), bifunctors (>=3.0), binary (>=0.5), deepseq (>=1.3), ghc-prim, hashable (>=, lens (>=3.9), QuickCheck (>=2), semigroups (>=0.18.3), strict (>=0.3.2 && <0.4) [details]
License BSD-3-Clause
Copyright (c) 2006-2008 by Roman Leshchinskiy (c) 2013-2014 by Simon Meier
Author Roman Leshchinskiy <>, Simon Meier <>
Maintainer Simon Meier <>
Category Data
Home page
Source repo head: git clone
Uploaded by SimonMeier at 2018-02-11T14:05:09Z
Distributions LTSHaskell:0.6.1, NixOS:0.6.1, Stackage:0.6.1
Downloads 8122 total (234 in the last 30 days)
Rating 2.0 (votes: 1) [estimated by Bayesian average]
Your Rating
  • λ
  • λ
  • λ
Status Hackage Matrix CI
Docs available [build log]
Last success reported on 2018-02-11 [all 1 reports]




Maintainer's Corner

For package maintainers and hackage trustees