While trying to compile the following code, which is enhanced version of read build on readMay from Safe package. readI :: (Typeable a, Read a) => String -> a readI str = case readMay str of Just x -> x Nothing -> error ("Prelude.read failed, expected type: " ++ (show (typ...
I can't figure out what the "| m -> w"-part means in a class definition like this: class (Monoid w, Monad m) => MonadWriter w m | m -> w What additional information does this add to the class definition? ...
I am trying to build a MaybeT-Transformer Monad, based on the example in the Real World Haskell, Chapter Monad Transformers: data MaybeT m a = MaybeT { runMT :: m (Maybe a) } instance (Monad m) => Monad (MaybeT m) where m >>= f = MaybeT $ do a <- runMT m case a of Just x -> runMT (...
Consider this example: applyKTimes :: Integral i => i -> (a -> a) -> a -> a applyKTimes 0 _ x = x applyKTimes k f x = applyKTimes (k-1) f (f x) applyThrice :: (a -> a) -> a -> a applyThrice = applyKTimes 3 The 3 in applyThrice is defaulted by GHC to an Integer as shown when compiling with -Wall: Warning: Defaulting the following con...
Say I have two type classes defined as follows that are identical in function but different in names: class Monad m where (>>=) :: m a -> (a -> m b) -> m b return :: a -> m a class PhantomMonad p where pbind :: p a -> (a -> p b) -> p b preturn :: a -> p b Is there a way to tie these two classes together so someth...
What are the similarities and the differences between Haskell's TypeClasses and Go's Interfaces? What are the relative merits / demerits of the two approaches? ...
When compiling my Haskell application with the -Wall option, GHC complains about orphaned instances, for example: Publisher.hs:45:9: Warning: orphan instance: instance ToSElem Result The type class ToSElem is not mine, it's defined by HStringTemplate. Now I know how to fix this (move the instance declaration into the module where...
Hi all, I have an ambiguous type variable error on the definition of "trial" below, I am wondering if there is anything that can be done to make this situation work? I want to really just deal with instances and not explicit data types (such as the MO1, MO2 included below). module Tc102 where class (Show a, Read a) => MyObj a where ...
What is the difference between Scala traits Haskell type class and C++0x Concepts? ...
I was messing around with Scala 2.8 for fun and trying to define a pimp which adds an "as" method to type constructors, allowing to convert from one functor to another (please overlook the fact that I'm not necessarily dealing with functors here). So for instance, you could use it like this: val array:Array[T] val list:List[T] = array.a...
Is it possible to implement Haskell typeclasses in C++? If yes, then how? ...
Hi, In order to grasp better typeclasses (starting pretty much form scratch) I had a go at modelling 2-D shapes with area calculations, like this: module TwoDShapes where class TwoDShape s where area :: s -> Float data Circle = Circle Float deriving Show aCircle radius | radius < 0 = error "circle radius must be non-negative" ...
Hi! I wish to say that a certain parametrized monad st works with a regular memory, but a subclass of my parametrized monad should impose an additional constraint on the type of memory. In code: class Memory m where ... class State st where unit :: Memory m => a -> st m m a bind :: (Memory m1, Memory m2, Memory m3) => st m1...
It normally seems the following is illegal: class Foo a where foo :: a -> b -> a Which makes sense; how do we know what b is? However, if we look at Functor's definition: class Functor f where fmap :: (a -> b) -> f a -> f b we see a and b showing up even though we only specify f as a type variable. I'm guessing this is all...
There seems to be a lot of enthusiasm among Scala bloggers lately for the type classes pattern, in which a simple class has functionality added to it by an additional class conforming to some trait or pattern. As a vastly oversimplified example, the simple class: case class Wotsit (value: Int) can be adapted to the Foo trait: trait F...
Hi I wrote this in scala and it won't compile: class TestDoubleDef{ def foo(p:List[String]) = {} def foo(p:List[Int]) = {} } the compiler notify: [error] double definition: [error] method foo:(List[String])Unit and [error] method foo:(List[Int])Unit at line 120 [error] have same type after erasure: (List)Unit I know JVM has no...
I have defined the following module to help me with FFI function export: {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, TypeSynonymInstances #-} module ExportFFI where import Foreign import Foreign.C class FFI basic ffitype | basic -> ffitype where toFFI :: basic -> IO ffitype fromFFI :: ffitype -> IO basic f...
Hi, I'm playing around in Haskell to try and get the hang of it. I'm running into problems with my typeclasses. What I'm trying to do is to make a generalized a* module by defining classes and methods and then I'm trying to use those in a program. My problem is that when I try to make a list of my Box data types an instance of my Board ...
Consider next declaration: :- type wrap(T) ---> wrap(T). :- inst wrap(I) ---> wrap(I). :- typeclass infer_wrap(A, B) <= ((A -> B)). :- instance infer_wrap(A, wrap(A)). Mercury (10.04) produces: monad.m:011: In instance declaration for `monad.infer_wrap(A, monad.wrap(A))': monad.m:011: the first arg is a type variable Even without...
haskell programmer. using F#. no typeclasses in F#. what to use when I need typeclasses? ...