newtype Questions
3
Solved
I am trying to use the newtype pattern to wrap a pre-existing type. That inner type has a modify method which lets us work with a borrowed mutable value in a callback:
struct Val;
struct Inner(Va...
2
Solved
What is the difference when I write this?
data Book = Book Int Int
versus
newtype Book = Book (Int, Int) -- "Book Int Int" is syntactically invalid
Councilwoman asked 4/5, 2011 at 20:50
1
Solved
I have a program which traverses an expression tree that does algebra on probability distributions, either sampling or computing the resulting distribution.
I have two implementations computing the...
Sita asked 1/7, 2022 at 23:47
2
Solved
In Haskell, there's two ways of providing an alias for types: type and newtype. type provides a type synonym, which means the synonym is regarded by the type checker as exactly the same as the orig...
1
Solved
I want to create a pair of newtypes Tag(str) and TagBuf(String),
analogous to how Path and PathBuf wrap OsStr and OsString. My
end goal is to have a map keyed by TagBuf and to be able to index into...
1
Solved
In Chapter 8 of thinking with types I learned that the fmap Sum part of
fastSum :: [Int] -> Int
fastSum = getSum . mconcat . fmap Sum
has a O(n) runtime cost, whereas using coerce instead avoid...
2
I am trying to define an instance:
newtype Join a = Join { getJoin :: a -> Bool }
deriving Generic
instance Monoid (Join a) where
f <> g = ???
mempty = ???
The goal is that the functi...
Ghat asked 19/7, 2020 at 16:22
3
Solved
Since learning Rust, I've become a fan of the newtype idiom which I gather Rust borrowed from Haskell.
A newtype is a distinct type based on a standard type which ensures that function parame...
2
Related question - Is it safe to derive MonadThrow, MonadCatch, MonadBaseControl, MonadUnliftIO, etc? - where I had enabled, both - DeriveAnyClass and GeneralizedNewtypeDeriving
to get the code to...
Corr asked 25/7, 2019 at 9:34
2
Solved
I have the following Haskell code, that compiles perfectly:
import Control.Monad.Reader (Reader (..))
import Data.Coerce (Coercible, coerce)
data Flow i o = Flow (i -> o) (o -> i)
coerceFl...
1
I'm trying to create my own data type, which will be part of Monad class, but
newtype Container a = Container a deriving Monad
gives me this error:
* Can't make a derived instance of `Monad Co...
2
I have a dozen of newtypes like this:
newtype MyBool = MyBool Bool
newtype MyInt = MyInt Int
I want to reuse existing instances:
instance MArray IOUArray Int IO where ...
instance MArray (STUAr...
5
Solved
I would like to know if it is bad form to do something like this:
data Alignment = LeftAl | CenterAl | RightAl
type Delimiter = Char
type Width = Int
setW :: Width -> Alignment -> Del...
Laburnum asked 16/7, 2019 at 16:48
2
Solved
Why does the WrappedMonad and WrappedArrow types exist? Is it because Monads were not Applicative? Given that WrappedArrow exists, should the instance
Arrow a => Applicative (Arrow a b)
sim...
Ludeman asked 12/2, 2015 at 1:52
1
Solved
I am learning Arrow following the tutorial programming with arrows. I've typed the following code according to the paper except that the SF is defined by data, not by newtype as in the paper (actua...
5
The Haskell programming language has a concept of newtypes: If I write newtype Foo = Foo (Bar), then a new type Foo is created that is isomorphic to Bar, i.e. there are bijective conversions betwee...
Shipp asked 11/7, 2013 at 10:41
1
Solved
How does this code
data D = D { _d :: ![P] } -- Note the strictness annotation!
Compare to this
newtype D = D { _d :: [P] }
An answer to a related question says:
the main difference betwee...
Buschi asked 20/11, 2018 at 10:42
5
Solved
Let's say I have the following newtype:
newtype Foo = Foo Integer deriving (Eq, Show)
Is there a concise way to add two Foo's:
(Foo 10) + (Foo 5) == Foo 15
or get the max:
max (Foo 10) (Foo 5)...
2
With Haskell 98 decls, the whole datatype must be either newtype or data. But data families can have a mix of newtype instance, data instance. Does that mean that being newtype is a property of the...
3
Solved
A pattern that presents itself the more often the more type safety is being introduced via newtype is to project a value (or several values) to a newtype wrapper, do some operations, and then retra...
0
I wanted a type for timestamped values that had an appropriate Semigroup instance (latest value wins). It turns out that Max (Arg UTCTime a) does exactly what I want, but is really awkward to work ...
1
Solved
I'm trying to create a pattern synonym for a newtype with an empty map.
{-# Language PatternSynonyms #-}
import qualified Data.Map as Map
newtype StoreEnv = StoreEnv (Map.Map Int String)
derivi...
Beebeebe asked 15/4, 2018 at 18:48
1
Solved
I've seen in some docs and tutorials:
runReader
runState
runState
What is the abstract concept that this pattern covers? What does running something mean in Haskell?
Side question, is there a ...
2
Solved
I have following type definition:
newtype Flip f a b =
Flip (f b a) deriving (Eq, Show)
Does the Flip data constructor has one or three arguments?
Consinder following implementation:
data K...
1
Solved
I'm going over continuations and I've come across two different approaches to structuring continuation types:
newtype C r a = C {runC :: (a -> r) -> r}
exampleFunction :: String -> C Boo...
Vang asked 17/2, 2017 at 8:39
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