Strict -template-haskell +package
A benchmarking library with a simple purpose: to strictly evaluate a value and report how long it takes.
Can be useful to identify the slow part of an algorithm, since Haskell's lazy evaluation can make it hard to see where the bottleneck lies.
This package provides strict versions of some standard Haskell data types (pairs, Maybe and Either). It also contains strict IO operations.
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 Strings 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 (http://hackage.haskell.org/packages/archive/lens/188.8.131.52/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 ...
> 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.
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.
This package provides head normal form strict versions of some standard Haskell concurrency abstractions (MVars,Chans), which provide control over where evaluation takes place not offered by the default lazy types. This may be useful for deciding when and where evaluation occurs, leading to improved time or space use, depending on the circumstances.
This plugin gives an example of defining a compiler plugin for GHC. You mark functions with the `Strictify` annotation and GHC makes the function strict (by recursively expanding non-recursive let bindings into case bindings.)
This library is a thin layer on top standard IO modules like System.IO and Data.IORef that re-expose these functions under a different type, namely SIO.
Find a local optimum of strictness annotations.
This is a strict version of the Get monad from the binary package. It's pretty much just a copy and paste job from the original source code. The binary team are currently unsure about their future plans w.r.t. strictness, so this is just a stop gap measure. See http://www.haskell.org/haskellwiki/DealingWithBinaryData for documentation.