<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">Constructors that bind existential variables were actually promoted pre-GHC 8.0, so they don't require -XTypeInType. In other words, your assumption is right.<div class=""><br class=""></div><div class="">Richard<br class=""><div class=""><br class=""><div class=""><div><blockquote type="cite" class=""><div class="">On May 4, 2018, at 1:36 AM, Tom Ellis <<a href="mailto:tom-lists-haskell-cafe-2013@jaguarpaw.co.uk" class="">tom-lists-haskell-cafe-2013@jaguarpaw.co.uk</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><span style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">Yes indeed, that's very helpful, thanks! The following *does* work so I</span><br style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">assume "existential types" do not fall under the purview of GADTs in this</span><br style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">context?</span><br style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><br style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><br style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><br style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">{-# LANGUAGE GADTs #-}</span><br style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">{-# LANGUAGE TypeFamilies #-}</span><br style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">{-# LANGUAGE DataKinds #-}</span><br style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">{-# LANGUAGE PolyKinds #-}</span><br style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><br style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">data T a b where</span><br style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class=""> E :: T a b -> T b c -> T a c</span><br style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><br style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">type family F (a :: T ka kb)</span><br style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><br style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">type instance F ('E a b) = ()</span><br style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><br style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><br style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">On Thu, May 03, 2018 at 06:16:13PM -0400, Richard Eisenberg wrote:</span><br style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><blockquote type="cite" style="font-family: Menlo-Regular; font-size: 11px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px;" class="">Data constructor I is a GADT constructor -- it constrains the values of its return type. Specifically, it says that T's two parameters must be the same. K is not a GADT constructor (it's just a GADT-syntax constructor), as it doesn't constraint the return type at all. In order to use a GADT constructor in a type, you need -XTypeInType; this ability was not available before GHC 8.0.<br class=""><br class="">There are concrete plans to smooth out this wrinkle:<span class="Apple-converted-space"> </span><a href="https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0020-no-type-in-type.rst" class="">https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0020-no-type-in-type.rst</a><span class="Apple-converted-space"> </span><<a href="https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0020-no-type-in-type.rst" class="">https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0020-no-type-in-type.rst</a>><br class=""><br class="">I hope this helps!<br class="">Richard<br class=""><br class=""><blockquote type="cite" class="">On May 3, 2018, at 5:24 PM, Tom Ellis <<a href="mailto:tom-lists-haskell-cafe-2013@jaguarpaw.co.uk" class="">tom-lists-haskell-cafe-2013@jaguarpaw.co.uk</a>> wrote:<br class=""><br class="">Can anyone explain why the type instance for K is fine but the one for I<br class="">needs TypeInType? (It does indeed work when I turn on TypeInType.)<br class=""><br class=""><br class=""><br class="">{-# LANGUAGE GADTs #-}<br class="">{-# LANGUAGE TypeFamilies #-}<br class="">{-# LANGUAGE DataKinds #-}<br class="">{-# LANGUAGE PolyKinds #-}<br class=""><br class="">data T a b where<br class="">K :: b -> T a b<br class="">I :: T a a<br class=""><br class="">type family F (a :: T ka kb)<br class=""><br class="">-- Data constructor ‘I’ cannot be used here<br class="">-- (Perhaps you intended to use TypeInType)<br class="">-- type instance F 'I = ()<br class=""><br class="">type instance F ('K b) = ()<br class="">_______________________________________________<br class="">Haskell-Cafe mailing list<br class="">To (un)subscribe, modify options or view archives go to:<br class=""><a href="http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe" class="">http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe</a><br class="">Only members subscribed via the mailman list are allowed to post.<br class=""></blockquote><br class=""></blockquote><br style="font-family: Menlo-Regular; 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