<html><head><meta http-equiv="Content-Type" content="text/html; charset=us-ascii"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class="">I think the general answer to your question is: no, you can't avoid this pattern match. In your particular case, the domain (Global Renamed) is a subset of the range (Global Generated), and so we can imagine a function that just changes the type without any fuss. This would, I'm pretty sure, be safe. But GHC has no notion of this kind of one-way transformation, so you're stuck just doing it via a manual pattern-match.<div class=""><br class=""></div><div class="">I hope this helps!</div><div class="">Richard<br class=""><div><br class=""><blockquote type="cite" class=""><div class="">On Jun 27, 2020, at 3:23 PM, chris done <<a href="mailto:haskell-cafe@chrisdone.com" class="">haskell-cafe@chrisdone.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div style="font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; color: rgb(31, 31, 31); font-family: "Proxima Nova", system-ui, -apple-system, "Segoe UI", Arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2; background-color: rgb(255, 255, 255);" class="">Hi all,<br class=""></div><div style="font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; color: rgb(31, 31, 31); font-family: "Proxima Nova", system-ui, -apple-system, "Segoe UI", Arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2; background-color: rgb(255, 255, 255);" class=""><br class=""></div><div style="font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; color: rgb(31, 31, 31); font-family: "Proxima Nova", system-ui, -apple-system, "Segoe UI", Arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2; background-color: rgb(255, 255, 255);" class="">I have stages in my compiler that convert e.g. Global Renamed -> Global Generated, etc. where certain constructors are available in certain stages.<span class=""> </span><br class=""></div><div style="font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; color: rgb(31, 31, 31); font-family: "Proxima Nova", system-ui, -apple-system, "Segoe UI", Arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2; background-color: rgb(255, 255, 255);" class=""><br class=""></div><div style="font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; color: rgb(31, 31, 31); font-family: "Proxima Nova", system-ui, -apple-system, "Segoe UI", Arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2; background-color: rgb(255, 255, 255);" class="">data GlobalRef s where<br class=""></div><div style="font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; color: rgb(31, 31, 31); font-family: "Proxima Nova", system-ui, -apple-system, "Segoe UI", Arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2; background-color: rgb(255, 255, 255);" class=""> FromIntegerGlobal :: GlobalRef s<br class=""></div><div style="font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; color: rgb(31, 31, 31); font-family: "Proxima Nova", system-ui, -apple-system, "Segoe UI", Arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2; background-color: rgb(255, 255, 255);" class=""> FromDecimalGlobal :: GlobalRef s<br class=""></div><div style="font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; color: rgb(31, 31, 31); font-family: "Proxima Nova", system-ui, -apple-system, "Segoe UI", Arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2; background-color: rgb(255, 255, 255);" class=""> InstanceGlobal :: !InstanceName -> GlobalRef Resolved<br class=""></div><div style="font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; color: rgb(31, 31, 31); font-family: "Proxima Nova", system-ui, -apple-system, "Segoe UI", Arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2; background-color: rgb(255, 255, 255);" class=""><br class=""></div><div style="font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; color: rgb(31, 31, 31); font-family: "Proxima Nova", system-ui, -apple-system, "Segoe UI", Arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2; background-color: rgb(255, 255, 255);" class="">E.g. after type class resolution is done, the InstanceGlobal constructor is available. But instances can't appear in the AST at other stages.<br class=""></div><div style="font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; color: rgb(31, 31, 31); font-family: "Proxima Nova", system-ui, -apple-system, "Segoe UI", Arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2; background-color: rgb(255, 255, 255);" class=""><br class=""></div><div style="font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; color: rgb(31, 31, 31); font-family: "Proxima Nova", system-ui, -apple-system, "Segoe UI", Arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2; background-color: rgb(255, 255, 255);" class="">In three stages, I've had to copy/paste this code: <br class=""></div><div style="font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; color: rgb(31, 31, 31); font-family: "Proxima Nova", system-ui, -apple-system, "Segoe UI", Arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2; background-color: rgb(255, 255, 255);" class=""><br class=""></div><div style="font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; color: rgb(31, 31, 31); font-family: "Proxima Nova", system-ui, -apple-system, "Segoe UI", Arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2; background-color: rgb(255, 255, 255);" class="">+ refl =<br class=""></div><div style="font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; color: rgb(31, 31, 31); font-family: "Proxima Nova", system-ui, -apple-system, "Segoe UI", Arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2; background-color: rgb(255, 255, 255);" class="">+ case name of<br class=""></div><div style="font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; color: rgb(31, 31, 31); font-family: "Proxima Nova", system-ui, -apple-system, "Segoe UI", Arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2; background-color: rgb(255, 255, 255);" class="">+ FromIntegerGlobal -> FromIntegerGlobal<br class=""></div><div style="font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; color: rgb(31, 31, 31); font-family: "Proxima Nova", system-ui, -apple-system, "Segoe UI", Arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2; background-color: rgb(255, 255, 255);" class="">+ FromDecimalGlobal -> FromDecimalGlobal<br class=""></div><div style="font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; color: rgb(31, 31, 31); font-family: "Proxima Nova", system-ui, -apple-system, "Segoe UI", Arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2; background-color: rgb(255, 255, 255);" class=""><br class=""></div><div style="font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; color: rgb(31, 31, 31); font-family: "Proxima Nova", system-ui, -apple-system, "Segoe UI", Arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2; background-color: rgb(255, 255, 255);" class="">Because if I just put `name` then GHC will complain that the types are different, which is correct. But a straight-forward pattern match gives GHC the proof that they can be converted.<br class=""></div><div style="font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; color: rgb(31, 31, 31); font-family: "Proxima Nova", system-ui, -apple-system, "Segoe UI", Arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2; background-color: rgb(255, 255, 255);" class=""><br class=""></div><div style="font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; color: rgb(31, 31, 31); font-family: "Proxima Nova", system-ui, -apple-system, "Segoe UI", Arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2; background-color: rgb(255, 255, 255);" class="">Is there a handy way already implemented that could derive either this code or this proof for me?<br class=""></div><div style="font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; color: rgb(31, 31, 31); font-family: "Proxima Nova", system-ui, -apple-system, "Segoe UI", Arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2; background-color: rgb(255, 255, 255);" class=""><br class=""></div><div style="font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; color: rgb(31, 31, 31); font-family: "Proxima Nova", system-ui, -apple-system, "Segoe UI", Arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2; background-color: rgb(255, 255, 255);" class="">Cheers,<br class=""></div><div style="font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; color: rgb(31, 31, 31); font-family: "Proxima Nova", system-ui, -apple-system, "Segoe UI", Arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2; background-color: rgb(255, 255, 255);" class=""><br class=""></div><div style="font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none; color: rgb(31, 31, 31); font-family: "Proxima Nova", system-ui, -apple-system, "Segoe UI", Arial, sans-serif; font-size: 14px; font-variant-ligatures: normal; orphans: 2; widows: 2; background-color: rgb(255, 255, 255);" class="">Chris<br class=""></div><span style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; 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="">_______________________________________________</span><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; 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