<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=""><br class=""><div><br class=""><blockquote type="cite" class=""><div class="">On Sep 3, 2020, at 1:51 PM, Iavor Diatchki <<a href="mailto:iavor.diatchki@gmail.com" class="">iavor.diatchki@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class="">How about the following rule: unlifted patterns are always strict (i.e., you desugar them as if they had an explicit `!`). A pattern is "unlifted" if the type it examines is unlifted. Seems simple enough and, I think, it would do what most folks would expect. </div></div></blockquote><div><br class=""></div><div>I don't think it's this simple. For example:</div><div><br class=""></div><div>> data X = MkX Int#</div><div>></div><div>> a = let MkX 3# = undefined in ()</div><div>> b = let MkX z = undefined in ()</div><div>> c = let MkX _ = undefined in ()</div><div>> d = let MkX {} = undefined in ()</div><div>> e = let _ :: X = undefined in ()</div><div><br class=""></div><div>Which of these diverge? e definitely converges, as X is lifted. b definitely diverges, because it binds z, a variable of an unlifted type, to a component of a diverging computation.</div><div><br class=""></div><div>In GHC today, all the cases other than b converge.</div><div><br class=""></div><div>Iavor suggests that a should diverge: 3# is a pattern of an unlifted type. What about c? What about d? Very unclear to me.</div><div><br class=""></div><div>Note that banging the pattern nested inside the MkX does not change the behavior (in GHC today) for any of the cases where this makes sense to do so.</div><div><br class=""></div><div>Richard</div><div><br class=""></div><blockquote type="cite" class=""><div class=""><div dir="ltr" class=""><div class=""><br class=""></div><div class="">I guess a more explicit option would be to make it an error to use a lazy pattern on an unlifted type, and require programmers to manually add the `!` but I am not sure that gains much, and is more work in the compiler.</div><div class=""><br class=""></div><div class="">-Iavor</div></div><br class=""><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Thu, Sep 3, 2020 at 10:38 AM Richard Eisenberg <<a href="mailto:rae@richarde.dev" class="">rae@richarde.dev</a>> wrote:<br class=""></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div style="overflow-wrap: break-word;" class=""><br class=""><div class=""><br class=""><blockquote type="cite" class=""><div class="">On Sep 3, 2020, at 12:10 PM, Spiwack, Arnaud <<a href="mailto:arnaud.spiwack@tweag.io" target="_blank" class="">arnaud.spiwack@tweag.io</a>> wrote:</div><br class=""><div class=""><span style="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;text-decoration:none;float:none;display:inline" class="">This is all a tad tricky, I must say.</span></div></blockquote></div><br class=""><div class="">... which is one of the reasons I originally wanted one simple rule. I'm not now saying I was in the right, but it is an attractive resting point for this discussion.</div><div class=""><br class=""></div><div class="">To be clear, I don't think there's going to be any concrete action here without a proposal, so perhaps once this thread finds a resting point different than the status quo, someone will have to write it up.</div><div class=""><br class=""></div><div class="">Richard</div></div>_______________________________________________<br class="">
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