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Strict variants of the types provided by base

Motivation

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 (http://hackage.haskell.org/package/vector) instead of strict lists. Moreover, instead of Strings one probably wants to use strict Text values from the text library (http://hackage.haskell.org/package/text).

Example

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 particular interest is the Strict type-class provided by the lens library (http://hackage.haskell.org/packages/archive/lens/3.9.0.2/doc/html/Control-Lens-Iso.html#t:Strict). 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 ...
  where
    spellDuration = M.fromList [("fireball", 5)]

See http://www.haskellforall.com/2013/05/program-imperatively-using-haskell.html for a gentle introduction to lenses and state manipulation.

Related work

Note that this package uses the types provided by the strict package (http://hackage.haskell.org/package/strict), but organizes them a bit differently. More precisely, the @strict-base-types@ package

  • only provides the fully strict variants of types from base,

  • is in-sync with the current base library (base-4.6),

  • provides the missing instances for (future) Haskell platform packages, and

  • conforms to the standard policy that strictness variants of an existing datatype are identified by suffixing 'Strict' or 'Lazy' in the module hierarchy.