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.
Make all fields of every constructor strict; i.e., add a bang to all fields.
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 'String's one probably wants to use strict 'Text' values
from the 'text' library (<http://hackage.haskell.org/package/text>).
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
> (-# 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
for a gentle introduction to lenses and state manipulation.
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