{"id":661,"date":"2022-08-30T15:05:04","date_gmt":"2022-08-30T15:05:04","guid":{"rendered":"https:\/\/unknownerror.org\/index.php\/2013\/11\/09\/problem-about-gadt-collection-of-common-programming-errors\/"},"modified":"2022-08-30T15:05:04","modified_gmt":"2022-08-30T15:05:04","slug":"problem-about-gadt-collection-of-common-programming-errors","status":"publish","type":"post","link":"https:\/\/unknownerror.org\/index.php\/2022\/08\/30\/problem-about-gadt-collection-of-common-programming-errors\/","title":{"rendered":"problem about gadt-Collection of common programming errors"},"content":{"rendered":"<ul>\n<li><img decoding=\"async\" src=\"http:\/\/www.gravatar.com\/avatar\/b9ee6de1ceebfa727c621a0941dc27c7?s=32&amp;d=identicon&amp;r=PG\" \/><br \/>\nkaosjester<br \/>\nhaskell typeclass gadt data-kinds<br \/>\nThe following definitons are kind of required to understand what I&#8217;m asking:data Param = PA | PB | PCdata R p a whereA :: S a -&gt; R PA (S a)B :: S a -&gt; R PB (S a)data S a wherePrim :: a -&gt; S aHO :: R pa a -&gt; R pb b -&gt; S ((R pa a) -&gt; (R pb b))Pair :: R pa a -&gt; R pb b -&gt; S ((R pa a), (R pb b))data Box r a = Box r a I would like to write a function using these definitions that works as follows:trans :: t -&gt; TransIn t -&gt; TransOut twhereTransIn (R &#8216;PA (S a)) = a TransIn (R &#8216;PB (S a)) = a TransIn (R &#8216;PA (S (r1, r2))) = (TransIn r1, TransIn r2) TransIn (R &#8216;PA (S (r1, r2))) = (TransIn r1, TransIn<\/li>\n<li><img decoding=\"async\" src=\"http:\/\/www.gravatar.com\/avatar\/1cb3b3d7e0ccce5f039c37f9bd85de11?s=32&amp;d=identicon&amp;r=PG\" \/><br \/>\nphynfo<br \/>\nhaskell ghc typeclass gadt<br \/>\nI have the following GADTdata Vec n a whereT :: Vec VZero a(:.) :: (VNat n) =&gt; a -&gt; (Vec n a) -&gt; (Vec (VSucc n) a)to model fixed length vectors, using the class class VNat ndata VZero instance VNat VZerodata VSucc n instance (VNat n) =&gt; VNat (VSucc n)I tried to programm an app<\/li>\n<li><img decoding=\"async\" src=\"http:\/\/www.gravatar.com\/avatar\/ac0f5806a57ad801fec9f5619763f667?s=32&amp;d=identicon&amp;r=PG\" \/><br \/>\nmasonk<br \/>\nhaskell gadt<br \/>\nI have this type and these functions:data Tag a whereTag :: (Show a, Eq a, Ord a, Storable a, Binary a) =&gt; a -&gt; BL.ByteString -&gt; Tag agetVal :: Tag a -&gt; a getVal (Tag v _) = visBigger :: Tag a -&gt; Tag a -&gt; Bool a `isBigger` b =<\/li>\n<li><img decoding=\"async\" src=\"http:\/\/www.gravatar.com\/avatar\/9f8f6670ea2f0b00e8b36a2dd5ad9be8?s=32&amp;d=identicon&amp;r=PG\" \/><br \/>\nAlessandro Vermeulen<br \/>\nhaskell dsl gadt<br \/>\nCan we transform a GADT without a given constraint on its constructors to a GADT that does have the said constraint? I want to do this because I want to get a deep-embedding of Arrows and do some interesting things with the representation that (for now) seem to require Typeable. (One reason)data DSL a b whereId :: DSL a aComp :: DSL b c -&gt; DSL a b -&gt; DSL a c&#8211;<\/li>\n<li><img decoding=\"async\" src=\"http:\/\/www.gravatar.com\/avatar\/b52bc71fb15db17f585b912aed45b217?s=32&amp;d=identicon&amp;r=PG\" \/><br \/>\ntibbe<br \/>\nhaskell dsl gadt<br \/>\nIn Type-Safe Observable Sharing in Haskell Andy Gill shows how to recover sharing that existed on the Haskell level, in a DSL. His solution is implemented in the data-reify package. Can this approach be modified to work with GADTs? For example, given this GADT:data Ast<\/li>\n<li><img decoding=\"async\" src=\"http:\/\/www.gravatar.com\/avatar\/1074f0a031cca12280359231a87662af?s=32&amp;d=identicon&amp;r=PG\" \/><br \/>\nEric<br \/>\nhaskell parameterized ambiguous gadt<br \/>\nI have a pretty straightforward function that takes a parameterized data type and returns the same type:{-# LANGUAGE ScopedTypeVariables #-}class IntegerAsType a wherevalue :: a -&gt; Integernewtype (Num a, IntegerAsType n) =&gt; PolyRing a n = PolyRing [a] deriving (Eq) normalize :: (Num a, IntegerAsType n) =&gt; (PolyRing a n) -&gt; (PolyRing a n) normalize r@(PolyRing xs) | (genericLength xs)<\/li>\n<li><img decoding=\"async\" src=\"http:\/\/www.gravatar.com\/avatar\/79ab43010f900324ac5e2de19a2916cb?s=32&amp;d=identicon&amp;r=PG\" \/><br \/>\nPetr Pudl\u00e1k<br \/>\nhaskell functor gadt bottom-type<br \/>\nWhen discussing Void on Haskell Libraries mailing list, there was this remark:Back in the day it used to be implemented by an unsafeCoerce at the behestof Conor McBride who didn&#8217;t want to pa<\/li>\n<li><img decoding=\"async\" src=\"http:\/\/www.gravatar.com\/avatar\/2fca12c2f74a2ec257c458d74029fc54?s=32&amp;d=identicon&amp;r=PG\" \/><br \/>\nJames Koppel<br \/>\nhaskell types gadt<br \/>\nConsider the following code:data (:+:) f g a = Inl (f a) | Inr (g a)data A data Bdata Foo l whereFoo :: Foo Adata Bar l whereBar :: Bar Btype Sig = Foo :+: Barfun :: Sig B -&gt; Int fun (Inr Bar) = 1Even though fun is an<\/li>\n<li><img decoding=\"async\" src=\"http:\/\/www.gravatar.com\/avatar\/79ab43010f900324ac5e2de19a2916cb?s=32&amp;d=identicon&amp;r=PG\" \/><br \/>\nPetr Pudl\u00e1k<\/li>\n<li><img decoding=\"async\" src=\"http:\/\/www.gravatar.com\/avatar\/419a88bdf20aeaa3fb2de53ca0f4d836?s=32&amp;d=identicon&amp;r=PG\" \/><br \/>\nyatima2975<br \/>\nhaskell ghc gadt derived-instances<br \/>\nSuppose I have the following code:{-# LANGUAGE GADTs, DeriveDataTypeable, StandaloneDeriving #-} import Data.Typeableclass Eq t =&gt; OnlyEq t class (Eq t, Typeable t) =&gt; BothEqAndTypeable tdata Wrapper a whereWrap :: BothEqAndTypeable a =&gt; a -&gt; Wrapper aderiving instance Eq (Wrapper a) deriving instance Typeable1 WrapperThen, the following instance declaration works, w<\/li>\n<li><img decoding=\"async\" src=\"http:\/\/www.gravatar.com\/avatar\/093b9e3ca5270fccd55113d1b9619c8a?s=32&amp;d=identicon&amp;r=PG\" \/><br \/>\nOren Ben-Kiki<br \/>\nhaskell type-inference gadt<br \/>\nI am trying to create complex data structures with composable logic. That is, the data structure has a generic format (essentially, a record with some fields whose type can change) and some generic functions. Specific structures have specific implementation of the generic functions.There are two approaches I tried. One is to use the type system (with type classes, type families, functional dependencies etc.). The other is creating my own &#8220;vtable&#8221; and using GADTs. Both methods fail in a similar way &#8211; there seems to be something basic I&#8217;m missing here. Or, perhaps, there a better more Haskell-ish way to do this?Here is the failing &#8220;typed&#8221; code:{-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE FunctionalDependencies #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE TemplateHaskell #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE TypeSynonymInstances #-}module Typed whereimport Control.Monad.State import Data.Lens.Lazy import Data.Lens.Template&#8211; Generic Block. data Block state ports = Block { _blockState :: state, _blockPorts :: ports }&#8211; For the logic we want to use, we need some state and ports. data LogicState = LogicState { _field :: Bool } data LogicPorts incoming outgoing =LogicPorts { _input :: incoming, _output :: outgoing }makeLenses [ &#8221;Block, &#8221;LogicState, &#8221;LogicPorts ]&#8211; We need to describe how to reach the needed state and ports, &#8212; and provide a piece of the logic. class LogicBlock block incoming outgoing | block -&gt; incoming, block -&gt; outgoing wherelogicState :: block ~ Block state ports =&gt; Lens state LogicStatelogicPorts :: block ~ Block state ports =&gt; Lens ports (LogicPorts incoming outgoing)convert :: block ~ Block state ports =&gt; incoming -&gt; State block outgoingrunLogic :: State block outgoingrunLogic = dostate<\/li>\n<\/ul>\n<p id=\"rop\"><small>Originally posted 2013-11-09 21:18:13. <\/small><\/p>","protected":false},"excerpt":{"rendered":"<p>kaosjester haskell typeclass gadt data-kinds The following definitons are kind of required to understand what I&#8217;m asking:data Param = PA | PB | PCdata R p a whereA :: S a -&gt; R PA (S a)B :: S a -&gt; R PB (S a)data S a wherePrim :: a -&gt; S aHO :: R pa [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-661","post","type-post","status-publish","format-standard","hentry","category-uncategorized"],"_links":{"self":[{"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/posts\/661","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/comments?post=661"}],"version-history":[{"count":0,"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/posts\/661\/revisions"}],"wp:attachment":[{"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/media?parent=661"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/categories?post=661"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/tags?post=661"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}