Questions tagged [deriving]
In Haskell, a derived instance is an instance declaration that is generated automatically in conjunction with a data or newtype declaration. The body of a derived instance declaration is derived syntactically from the definition of the associated type.
deriving
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Deriving FromRow on a type with [String] field
I am building a haskell server with a postgres database. I am using postgresql-simple to query the db.
I have the following type:
data Reading = Reading
{ id :: UUID
, title :: String
, authors ...
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Deriving `Eq` for a data type based on a type family in Haskell
I'm working with type-level programming in Haskell and have encountered an issue with deriving an Eq instance for a data type based on a type family.
Here's a simplified version of my code:
{-# ...
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Deriving newtype recursively like functionality
Lets say, in package P, I've got a type A, defined something like so:
newtype A = A Int
There's then external package X (which I don't control, unlike P and Q), which has a class C and an instance:
...
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Which kinds of lawfull `Functor`s are possible in Haskell?
I know following types of Functors:
Polynomial functor == simple not-nested parameterized datatype
Generalization of 1, by using "parameter removing" newtypes: Fix, Some (existentials), ...
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Trying to add deriving(Show, Read) to an expression tree
I am fairly new to Haskell so it is probably something simple that I am missing but I have an expression tree that looks like this:
data Expression = Lit Float
| Add Expression ...
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Derive typeclass instances for opaque types in Scala 3
Is there a way in Scala 3 to use derives keyword in combination with opaque type aliases?
It would be nice to have a boilerplate-free way to provide a typeclass instance to a given opaque type alias ...
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How to use deriving Show() in mutually recursive types?
Let's directly see the codes.
type symbol =
| JumpDes of int
| CallDes of func
| StarDes of exp (*here needs the definition of type exp*)
deriving (Show)
type exp =
| Const of const
|...
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Why automatical deriving for newtype does not work
I use library msgpack-types-0.0.4 and I have a type:
newtype A = A Text
and I tried deriving MessagePack, deriving newtype MessagePack, standalone derivings, it seems nothing works (or I do something ...
- 3,645
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2
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244
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Problem when deriving Eq from data type with function as field in Haskell
I'm trying to deriving Eq from data type with a function as a field but doesn't work as expected.
I also try to write te instance but still doesn't work
data Conf = Conf {
rule :: ([Char] -> ...
4
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2
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252
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Can I use DerivingVia to derive instances for data types isomorphic to tuples
Given the following data type
data Both a b = Both { left :: a, right :: b }
I can write instances for Applicative etc. like so (omitting Functor here as we can use DeriveFunctor):
instance Monoid a =...
- 1,256
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Alternative of Show that only uses name
Is there something like Show (deriving Show) that only uses an algebraic datatype's constructors? (please don't mind that I'm using the word constructor, I don't know the right name...)
The reason for ...
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1
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Deriving via ReaderT
I'm writing the Monad instance for this type using ReaderT and deriving via
newtype HIO a = HIO {runHIO :: Set HiPermission -> IO a}
I've tried to do it this way
newtype HIO a = HIO {runHIO :: Set ...
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Using "deriving via" with a type family
I have a typeclass with a default implementation, and would like to provide a simple way to derive the typeclass if a user wants to use their custom monad.
Here's a solution someone else provided me:
{...
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Deriving from std::vector - what am I doing wrong? [duplicate]
Trying to switch from some custom made classes I built to std classes. Just for a start, I am trying to derive from std::vector.
#include <vector>
template < class DATA_T >
class CArray : ...
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Deriving a class instance for a type based on another class it's an instance of
I have a situation where I have some similar newtypes that all need to be instances of Random, Arbitrary, and lots of other stuff. They all need the same custom implementation of the functions randomR,...
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