<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><br class=""><div><br class=""><blockquote type="cite" class=""><div class="">On Dec 3, 2020, at 11:58 AM, Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com" class="">simonpj@microsoft.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><span style="caret-color: rgb(0, 0, 0); font-family: Calibri, sans-serif; font-size: 14.666666984558105px; 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; text-decoration: none; float: none; display: inline !important;" class="">Yes, but there is no prospect (that I know of) of a substantial improvement in [type inference for GADTs] – and what we have does not seem to cause problems in practice. And they are jolly useful and popular!</span></div></blockquote></div><br class=""><div class="">The problem I described would arise when someone who does not know about GADTs and type inference accidentally writes a GADT. But this cannot happen easily today, precisely because of the need to write the extension.</div><div class=""><br class=""></div><div class="">Useful, popular, and stable all help argue for an extension (and I agree here!), but I'm more concerned about error messages and the beginner experience, captured in our Criterion 2 of <a href="https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0372-ghc-extensions.rst#criteria" class="">https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0372-ghc-extensions.rst#criteria</a>.</div><div class=""><br class=""></div><div class="">Richard</div></body></html>