ifcxt alternatives and similar packages
Based on the "Control" category.
Alternatively, view ifcxt alternatives based on common mentions on social networks and blogs.

transient
A full stack, reactive architecture for general purpose programming. Algebraic and monadically composable primitives for concurrency, parallelism, event handling, transactions, multithreading, Web, and distributed computing with complete deinversion of control (No callbacks, no blocking, pure state) 
distributedclosure
Serializable closures for distributed programming. 
classyprelude
Type classes for mapping, folding, and traversing monomorphic containers 
auto
Haskell DSL and platform providing denotational, compositional api for discretestep, locally stateful, interactive programs, games & automations. http://hackage.haskell.org/package/auto 
classypreludeyesod
Type classes for mapping, folding, and traversing monomorphic containers 
extensibleeffects
Extensible Effects: An Alternative to Monad Transformers 
selective
Selective Applicative Functors: Declare Your Effects Statically, Select Which to Execute Dynamically 
ComonadSheet
A library for expressing "spreadsheetlike" computations with absolute and relative references, using fixedpoints of ndimensional comonads. 
these
An eitherorboth data type, with corresponding hybrid error/writer monad transformer. 
abstractpar
Type classes generalizing the functionality of the 'monadpar' library. 
hask
Category theory for Haskell with a lens flavor (you need GHC 7.8.3, not 7.8.2 to build this!) 
transientuniverse
A Cloud monad based on transient for the creation of Web and reactive distributed applications that are fully composable, where Web browsers are first class nodes in the cloud 
cloudhaskell
This is an umbrella development repository for Cloud Haskell 
distributedfork
A distributed data processing framework in Haskell. 
distributedprocessplatform
DEPRECATED (Cloud Haskell Platform) in favor of distributedprocessextras, distributedprocessasync, distributedprocessclientserver, distributedprocessregistry, distributedprocesssupervisor, distributedprocesstask and distributedprocessexecution 
monadvalidate
(NOTE: REPOSITORY MOVED TO NEW OWNER: https://github.com/lexilambda/monadvalidate) A Haskell monad transformer library for data validation 
monadcontrol
Lift control operations, like exception catching, through monad transformers 
operational
Implement monads by specifying instructions and their desired operational semantics. 
freereffects
An implementation of "Freer Monads, More Extensible Effects". 
monadtime
Type class for monads which carry the notion of the current time. 
effectmonad
Provides 'graded monads' and 'parameterised monads' to Haskell, enabling finegrained reasoning about effects. 
ixmonad
Provides 'graded monads' and 'parameterised monads' to Haskell, enabling finegrained reasoning about effects.
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Do you think we are missing an alternative of ifcxt or a related project?
README
IfCxt
This package introduces the function:
ifCxt :: IfCxt cxt => proxy cxt > (cxt => a) > a > a
This function acts like an if
statement where the proxy cxt
parameter is the condition.
If the type checker can satisfy the cxt
constraint, then the second argument cxt => a
is returned;
otherwise, the third argument a
is returned.
Before seeing more details about how ifCxt
is implemented,
let's look at three examples of how to use it.
Example 1: show every type
The cxtShow
function below is polymorphic over the type a
.
If a
is an instance of Show
, then cxtShow a
evaluates to show a
;
but if a
is not an instance of Show
, cxtShow a
evaluates to <<unshowable>>
.
cxtShow :: forall a. IfCxt (Show a) => a > String
cxtShow a = ifCxt (Proxy::Proxy (Show a))
(show a)
"<<unshowable>>"
In ghci:
ghci> cxtShow (1 :: Int)
"1"
ghci> cxtShow (id :: a > a)
"<<unshowable>>"
Example 2: make your code asymptotically efficient
The nub
function removes duplicate elements from lists.
It can be defined as:
nub :: Eq a => [a] > [a]
nub [] = []
nub (x:xs) = x : nub (filter (x/=) xs)
This function takes time O(n^{2).}
But if we also have an Ord
constraint, we can define a much more efficient version that takes time O(n log n):
nubOrd :: Ord a => [a] > [a]
nubOrd = go . sort
where
go (x1:x2:xs)
 x1==x2 = go (x2:xs)
 otherwise = x1 : go (x2:xs)
go [x] = [x]
go [] = []
Now, we can use the ifCxt
function to define a version of nub
that will automatically select the most efficient implementation for whatever type we happen to run it on:
cxtNub :: forall a. (Eq a, IfCxt (Ord a)) => [a] > [a]
cxtNub = ifCxt (Proxy::Proxy (Ord a)) nubOrd nub
Example 3: make your code numerically stable
The simplest way to sum a list of numbers is:
sumSimple :: Num a => [a] > a
sumSimple = foldl' (+) 0
This method has numerical stability issues on floating point representations. Kahan summation is a more accurate technique shown below:
sumKahan :: Num a => [a] > a
sumKahan = snd . foldl' go (0,0)
where
go (c,t) i = ((t't)y,t')
where
y = ic
t' = t+y
Because Kahan summation does a lot more work than simple summation, we would prefer not to run it on nonfloating point types.
The sumCxt
function below accomplishes this:
cxtSum :: forall a. (Num a, IfCxt (Floating a)) => [a] > a
cxtSum = ifCxt (Proxy::Proxy (Floating a)) sumKahan sumSimple
Notice that the ifCxt
function is conditioning on the Floating a
constraint,
which isn't actually used by the sumKahan
function.
How it works
The magic of the technique is in the IfCxt
class:
class IfCxt (cxt :: Constraint) where
ifCxt :: proxy cxt > (cxt => a) > a > a
(Notice that making a constraint an instance of a class requires theConstraintKinds
extension,
and the higher order (cxt => a)
parameter requires the RankNTypes
extension.)
There is a "global" instance defined as:
instance {# OVERLAPPABLE #} IfCxt cxt where ifCxt _ t f = f
What this says is that if no more specific instance is available, then the "global" ifCxt
function will be used, which always returns the f
(false) parameter.
Then for every instance of every other class, we need to define an overlapping IfCxt
instance that always returns the t
(true) parameter.
For example, for Show Int
, we define:
instance {# OVERLAPS #} IfCxt (Show Int) where ifCxt _ t f = t
This is a lot of boilerplate, so the template haskell function mkIfCxtInstances
can be used to define these instances automatically.
Unfortunately, due to a bug in template haskell we cannot enumerate all the classes currently in scope.
So you must manually call mkIfCxtInstances
on each class you want ifCxt
to work with.