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Questions tagged [ghc-generics]

Generic programming support in GHC allows defining classes with methods that do not need a user specification when instantiating: the method body is automatically derived by GHC. This is similar to what happens for standard classes such as Read and Show, for instance, but now for user-defined classes.

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Infinite recursion when enumerating all values of a Generic instance

For another answer of mine, I wrote the following code, providing diagonally traversed Universe instances for enumerable Generics (it's slightly updated from the version there, but uses the same logic): {-# LANGUAGE DeriveGeneric, TypeOperators,…
phipsgabler
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How to construct generic Functor instances using GHC.Generics (or other similar frameworks)?

I'm trying to learn GHC Generics. After reviewing several examples, I wanted to try to create a generic Functor instances (disregarding that GHC can derive them automatically for me). However, I realized I have no idea how to work with a…
Petr
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Advantages of SYB (scrap your boilerplate) over GHC Generics

Are any tasks that are possible only with SYB, or are much easier with it, when compared to GHC Generics?
Petr
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Implement Applicative builder style with Generics

Context If we have data Foo = Foo { x :: Maybe Int, y :: Maybe Text } we can already build it up applicative-style in an Applicative context (here IO) as myfoo :: IO Foo myfoo = Foo <$> getEnvInt "someX" <*> getEnvText "someY" Problem What if one…
ron
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Deriving projection functions using `generics-sop`

How would I go about deriving the function getField :: (Generic a, HasDatatypeInfo a) => Proxy (name :: Symbol) -> a -> b to project a field from an arbitrary record using a type-level string (Symbol), using the generics-sop library? This is…
Ari Fordsham
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Would it be possible to derive Data.Vector.Unbox via GHC's generic deriving?

It's possible to derive Storable via GHC's generic deriving mechanism: http://hackage.haskell.org/package/derive-storable (and https://hackage.haskell.org/package/derive-storable-plugin for performance). The only library I can find for deriving…
LogicChains
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How to derive generic traversals that involve a type family

When configuring our applications, often the way that field is defined is the same as the way the field is used: data CfgMyHostName = CfgMyHostName Text Other times, they differ. Let's make this formal in a typeclass: data UsagePhase = ConfigTime |…
ImAlsoGreg
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Using type families and Generics to find an Id value

This question is related to this one, where I wanted to avoid the boilerplate of extracting an Id value from a data structure, but in a type-safe manner. I'll repeat the relevant details of the problem here: suppose you have a type Id: newtype Id =…
Damian Nadales
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Retrieving record function in generic SOP

In Sum of Products approach, how would one retrieve the record function? An example code below with record datatype (ghc 7.10.3): {-# LANGUAGE DeriveGeneric #-} import qualified GHC.Generics as GHC import Generics.SOP data Rec = Rec { frec :: Int,…
Sal
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How do I create a ListIsomorphic instance for generic vectors?

Given the following class: class ListIsomorphic l where toList :: l a -> [a] fromList :: [a] -> l a How can I write a instance for vector types using Data.Vector.Generic? This doesn't work: instance (V.Vector v a) => ListIsomorphic v…
MaiaVictor
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Deriving functor instance, not on last type argument

Related to this question I asked earlier today. I have an AST data type with a large number of cases, which is parameterized by an "annotation" type data Expr ann def var = Plus a Int Int | ... | Times a Int Int deriving (Data, Typeable,…
jmite
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Recover type definitions using GHC.Generics

Yesterday I took a swing at trying to answer this question about a representation for a datatype. using GHC.Generics. I could recover type definitions for the example problem given, for example, for: data Artist = Artist Text Genre data Genre = Jazz…
Cirdec
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How do I replace Data.Generics with GHC.Generics?

So I've used syb for a long time, and often have functions like friendlyNames :: Data a => a -> a friendlyNames = everywhere (mkT (\(Name x _) -> Name x NameS)) What is the equivalent of this using GHC.Generics, assuming Generic a?
David Fox
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No instance for (Generic (f a)) arising from a use of `from'

I'm having issues with finding suitable type constraints for the following code {-# LANGUAGE DeriveGeneric #-} {-# LANGUAGE DefaultSignatures #-} {-# LANGUAGE TypeOperators #-} {-# LANGUAGE FlexibleContexts #-} import GHC.Generics data Value = One…
sgillis
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"packageName" with GHC.Generics

I have a class that provides a globally unique identifier for types: class Named a where nameOf :: a -> (String,String,String) -- (Package, Module, Identifier) default nameOf :: (Generic a, Named' (Rep a)) => a -> (String,String,String) nameOf =…
sam boosalis
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