darcs-2.18.5: a distributed, interactive, smart revision control system

Copyright | (c) Carter Schonwald 2013
License | BSD3, see license file
Maintainer | libraries@haskell.org
Stability | experimental
Portability | portable
Safe Haskell | None
Language | Haskell2010

Darcs.Util.StrictIdentity

Description

Synopsis

- newtype StrictIdentity a = StrictIdentity {
  - runStrictIdentity_ :: a
  }
- runStrictIdentity :: StrictIdentity a -> a

# Documentation

newtype StrictIdentity a #

`StrictIdentity` is a newtype wrapper for a given type `a` that satisfies the `Functor`, `Applicative`, and `Monad` laws when restricted to terminating strict computations.

The typical use case is to provide a light weight strict nested let notation for code that otherwise must use nested case expressions as a proxy for a strict let.

the general pattern is to write code of the form

    foo f h g x y z = runStrictIdentity $! do
        w <- return $! f x y
        j <- return $! h w z
        res <- return $! g w j
        return res

An example usage of `StrictIdentity` that compiles to assembly comparable to C is the following:

    (>>) = unsafeShiftR
    (<<) = unsafeShiftL
    outerShuffle64A :: Word -> Word
    outerShuffle64A !x =
        runStrictIdentity $! do
            x <- return $! ((x .&. 0x00000000FFFF0000) << 16 )
                .|. ((x>>16) .&. 0x00000000FFFF0000) .|. (x .&. 0xFFFF00000000FFFF)
            x <-  return $! ((x .&. 0x0000FF000000FF00 ) <<  8 )
                .|. (x >> 8) .&. 0x0000FF000000FF00 .|. (x  .&. 0xFF0000FFFF0000FF)
            x<-  return $! (( x .&. 0x00F000F000F000F0 ) << 4 )
                .|. (x >> 4) .&. 0x00F000F000F000F0 .|. (x .&. 0xF00FF00FF00FF00F )
            x<-   return $!((x .&.  0x0C0C0C0C0C0C0C0C )<< 2 )
                .|. (x >> 2) .&. 0x0C0C0C0C0C0C0C0C .|.( x .&. 0xC3C3C3C3C3C3C3C3)
            x<-   return $! ( (x .&. 0x2222222222222222)  << 1 )
                .|. (x>> 1) .&. 0x2222222222222222 .|. (x .&. 0x9999999999999999)
            return x

Constructors

StrictIdentity |

Fields

- runStrictIdentity_ :: a

#### Instances

Instances details

Applicative StrictIdentity # |
Instance details Defined in Darcs.Util.StrictIdentity Methods pure :: a -> StrictIdentity a # (<*>) :: StrictIdentity (a -> b) -> StrictIdentity a -> StrictIdentity b # liftA2 :: (a -> b -> c) -> StrictIdentity a -> StrictIdentity b -> StrictIdentity c # (*>) :: StrictIdentity a -> StrictIdentity b -> StrictIdentity b # (<*) :: StrictIdentity a -> StrictIdentity b -> StrictIdentity a #
Functor StrictIdentity # |
Instance details Defined in Darcs.Util.StrictIdentity Methods fmap :: (a -> b) -> StrictIdentity a -> StrictIdentity b # (<$) :: a -> StrictIdentity b -> StrictIdentity a #
Monad StrictIdentity # |
Instance details Defined in Darcs.Util.StrictIdentity Methods (>>=) :: StrictIdentity a -> (a -> StrictIdentity b) -> StrictIdentity b # (>>) :: StrictIdentity a -> StrictIdentity b -> StrictIdentity b # return :: a -> StrictIdentity a #
MonadFix StrictIdentity # |
Instance details Defined in Darcs.Util.StrictIdentity Methods mfix :: (a -> StrictIdentity a) -> StrictIdentity a #

runStrictIdentity :: StrictIdentity a -> a #

`runStrictIdentity` unwraps a value of type ``StrictIdentity` ty` into a value of type `ty`, strictly.