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Simon Peyton Jones pushed to branch wip/T23070 at <a href="https://gitlab.haskell.org/ghc/ghc">Glasgow Haskell Compiler / GHC</a>
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<strong style="font-weight: bold;"><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/8357a7db55ced7e9f8f42c7ab77bc1408b0599a1">8357a7db</a></strong>
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<span> by Simon Peyton Jones </span> <i> at 2023-03-04T10:22:12+00:00 </i>
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compiler/GHC/Tc/Solver/Dict.hs
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compiler/GHC/Tc/Solver/Equality.hs
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<a href="https://gitlab.haskell.org/ghc/ghc/-/commit/8357a7db55ced7e9f8f42c7ab77bc1408b0599a1#60f69bb5bc0ef8edf7d13518a1e48f7d36236318"><strong style="font-weight: bold;">compiler/GHC/Tc/Solver/Dict.hs</strong></a>
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<td class="line_content match" style="color: rgba(31,30,36,0.24); padding: inherit;" bgcolor="#fbfafd">@@ -39,7 +39,6 @@ import GHC.Driver.Session</td>
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<td class="line_content" style="padding: inherit;"><pre style='display: block; font-size: 0.875rem; color: #333238; position: relative; font-family: var(--default-mono-font, "Menlo"),"DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'> <span id="LC39" class="line" lang="haskell"></span>
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<td class="new_line diff-line-num" data-linenumber="40" style="width: 35px; color: rgba(31,30,36,0.24); border-right-width: 1px; border-right-color: #ececef; border-right-style: solid; padding: inherit;" align="right" bgcolor="#fbfafd">
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<td class="line_content" style="padding: inherit;"><pre style='display: block; font-size: 0.875rem; color: #333238; position: relative; font-family: var(--default-mono-font, "Menlo"),"DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'> <span id="LC40" class="line" lang="haskell"><span class="kr" style="font-weight: 600;">import</span> <span class="k" style="font-weight: 600;">qualified</span> <span class="nn" style="color: #555;">GHC.LanguageExtensions</span> <span class="k" style="font-weight: 600;">as</span> <span class="n" style="color: #333;">LangExt</span></span>
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<td class="line_content" style="padding: inherit;"><pre style='display: block; font-size: 0.875rem; color: #333238; position: relative; font-family: var(--default-mono-font, "Menlo"),"DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'> <span id="LC41" class="line" lang="haskell"></span>
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<td class="old_line diff-line-num old" data-linenumber="42" style="width: 35px; color: rgba(31,30,36,0.24); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: inherit;" align="right" bgcolor="#f9d7dc">
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<td class="new_line diff-line-num old" data-linenumber="42" style="width: 35px; color: rgba(31,30,36,0.24); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: inherit;" align="right" bgcolor="#f9d7dc">
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<td class="line_content old" style="padding: inherit;" bgcolor="#fbe9eb"><pre style='display: block; font-size: 0.875rem; color: #333238; position: relative; font-family: var(--default-mono-font, "Menlo"),"DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'>-<span id="LC42" class="line" lang="haskell"><span class="kr" style="font-weight: 600;">import</span> <span class="nn" style="color: #555;">Data.List</span><span class="p">(</span> <span class="n" style="color: #333;">deleteFirstsBy</span> <span class="p">)</span></span>
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<td class="new_line diff-line-num" data-linenumber="42" style="width: 35px; color: rgba(31,30,36,0.24); border-right-width: 1px; border-right-color: #ececef; border-right-style: solid; padding: inherit;" align="right" bgcolor="#fbfafd">
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<td class="line_content" style="padding: inherit;"><pre style='display: block; font-size: 0.875rem; color: #333238; position: relative; font-family: var(--default-mono-font, "Menlo"),"DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'> <span id="LC42" class="line" lang="haskell"><span class="kr" style="font-weight: 600;">import</span> <span class="nn" style="color: #555;">Data.Maybe</span> <span class="p">(</span> <span class="nf" style="color: #900; font-weight: 600;">listToMaybe</span><span class="p">,</span> <span class="nf" style="color: #900; font-weight: 600;">mapMaybe</span> <span class="p">)</span></span>
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<a href="https://gitlab.haskell.org/ghc/ghc/-/commit/8357a7db55ced7e9f8f42c7ab77bc1408b0599a1#03ec4f8741105271af29a61fdb47c8feba9bbcfa"><strong style="font-weight: bold;">compiler/GHC/Tc/Solver/Equality.hs</strong></a>
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<td class="line_content match" style="color: rgba(31,30,36,0.24); padding: inherit;" bgcolor="#fbfafd">@@ -244,7 +244,7 @@ can_eq_nc' False rdr_env envs ev eq_rel _ ps_ty1 _ ps_ty2</td>
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<td class="line_content" style="padding: inherit;"><pre style='display: block; font-size: 0.875rem; color: #333238; position: relative; font-family: var(--default-mono-font, "Menlo"),"DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'> <span id="LC244" class="line" lang="haskell"> <span class="p">;</span> <span class="n" style="color: #333;">can_eq_nc'</span> <span class="kt" style="color: #458; font-weight: 600;">True</span> <span class="n" style="color: #333;">rdr_env</span> <span class="n" style="color: #333;">envs</span> <span class="n" style="color: #333;">new_ev</span> <span class="n" style="color: #333;">eq_rel</span> <span class="n" style="color: #333;">xi1</span> <span class="n" style="color: #333;">xi1</span> <span class="n" style="color: #333;">xi2</span> <span class="n" style="color: #333;">xi2</span> <span class="p">}</span></span>
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<td class="line_content" style="padding: inherit;"><pre style='display: block; font-size: 0.875rem; color: #333238; position: relative; font-family: var(--default-mono-font, "Menlo"),"DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'> <span id="LC246" class="line" lang="haskell"><span class="c1" style="color: #998; font-style: italic;">----------------------------</span></span>
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<td class="line_content old" style="padding: inherit;" bgcolor="#fbe9eb"><pre style='display: block; font-size: 0.875rem; color: #333238; position: relative; font-family: var(--default-mono-font, "Menlo"),"DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'>-<span id="LC247" class="line" lang="haskell"><span class="c1" style="color: #998; font-style: italic;">-- Look for a canonical LHS.<span class="idiff left right deletion" style="background-color: #fac5cd;"> See Note [Canonical LHS].</span></span></span>
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<td class="line_content new" style="padding: inherit;" bgcolor="#ecfdf0"><pre style='display: block; font-size: 0.875rem; color: #333238; position: relative; font-family: var(--default-mono-font, "Menlo"),"DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'>+<span id="LC247" class="line" lang="haskell"><span class="c1" style="color: #998; font-style: italic;">-- Look for a canonical LHS.</span></span>
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<td class="new_line diff-line-num" data-linenumber="248" style="width: 35px; color: rgba(31,30,36,0.24); border-right-width: 1px; border-right-color: #ececef; border-right-style: solid; padding: inherit;" align="right" bgcolor="#fbfafd">
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<td class="line_content" style="padding: inherit;"><pre style='display: block; font-size: 0.875rem; color: #333238; position: relative; font-family: var(--default-mono-font, "Menlo"),"DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'> <span id="LC248" class="line" lang="haskell"><span class="c1" style="color: #998; font-style: italic;">-- Only rewritten types end up below here.</span></span>
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<td class="line_content" style="padding: inherit;"><pre style='display: block; font-size: 0.875rem; color: #333238; position: relative; font-family: var(--default-mono-font, "Menlo"),"DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'> <span id="LC249" class="line" lang="haskell"><span class="c1" style="color: #998; font-style: italic;">----------------------------</span></span>
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<td class="line_content" style="padding: inherit;"><pre style='display: block; font-size: 0.875rem; color: #333238; position: relative; font-family: var(--default-mono-font, "Menlo"),"DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'> <span id="LC250" class="line" lang="haskell"></span>
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<td class="diff-line-num unfold js-unfold old_line" data-linenumber="2497" style="width: 35px; color: rgba(31,30,36,0.24); border-right-width: 1px; border-right-color: #ececef; border-right-style: solid; padding: inherit;" align="right" bgcolor="#fbfafd">...</td>
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<td class="line_content match" style="color: rgba(31,30,36,0.24); padding: inherit;" bgcolor="#fbfafd">@@ -2497,10 +2497,6 @@ We avoid this problem by orienting the resulting given so that the unification</td>
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2497
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<td class="new_line diff-line-num" data-linenumber="2497" style="width: 35px; color: rgba(31,30,36,0.24); border-right-width: 1px; border-right-color: #ececef; border-right-style: solid; padding: inherit;" align="right" bgcolor="#fbfafd">
2497
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<td class="line_content" style="padding: inherit;"><pre style='display: block; font-size: 0.875rem; color: #333238; position: relative; font-family: var(--default-mono-font, "Menlo"),"DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'> <span id="LC2497" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">variable is on the left (note that alternatively we could attempt to</span></span>
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<td class="old_line diff-line-num" data-linenumber="2498" style="width: 35px; color: rgba(31,30,36,0.24); border-right-width: 1px; border-right-color: #ececef; border-right-style: solid; padding: inherit;" align="right" bgcolor="#fbfafd">
2498
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<td class="new_line diff-line-num" data-linenumber="2498" style="width: 35px; color: rgba(31,30,36,0.24); border-right-width: 1px; border-right-color: #ececef; border-right-style: solid; padding: inherit;" align="right" bgcolor="#fbfafd">
2498
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<td class="line_content" style="padding: inherit;"><pre style='display: block; font-size: 0.875rem; color: #333238; position: relative; font-family: var(--default-mono-font, "Menlo"),"DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'> <span id="LC2498" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">enforce this at canonicalization).</span></span>
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<td class="old_line diff-line-num" data-linenumber="2499" style="width: 35px; color: rgba(31,30,36,0.24); border-right-width: 1px; border-right-color: #ececef; border-right-style: solid; padding: inherit;" align="right" bgcolor="#fbfafd">
2499
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<td class="new_line diff-line-num" data-linenumber="2499" style="width: 35px; color: rgba(31,30,36,0.24); border-right-width: 1px; border-right-color: #ececef; border-right-style: solid; padding: inherit;" align="right" bgcolor="#fbfafd">
2499
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<td class="line_content" style="padding: inherit;"><pre style='display: block; font-size: 0.875rem; color: #333238; position: relative; font-family: var(--default-mono-font, "Menlo"),"DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'> <span id="LC2499" class="line" lang="haskell"></span>
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<td class="old_line diff-line-num old" data-linenumber="2500" style="width: 35px; color: rgba(31,30,36,0.24); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: inherit;" align="right" bgcolor="#f9d7dc">
2500
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<td class="new_line diff-line-num old" data-linenumber="2500" style="width: 35px; color: rgba(31,30,36,0.24); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: inherit;" align="right" bgcolor="#f9d7dc">
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<td class="line_content old" style="padding: inherit;" bgcolor="#fbe9eb"><pre style='display: block; font-size: 0.875rem; color: #333238; position: relative; font-family: var(--default-mono-font, "Menlo"),"DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'>-<span id="LC2500" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">See also Note [No touchables as FunEq RHS] in GHC.Tc.Solver.Monad; avoiding</span></span>
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<td class="old_line diff-line-num old" data-linenumber="2501" style="width: 35px; color: rgba(31,30,36,0.24); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: inherit;" align="right" bgcolor="#f9d7dc">
2501
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<td class="new_line diff-line-num old" data-linenumber="2500" style="width: 35px; color: rgba(31,30,36,0.24); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: inherit;" align="right" bgcolor="#f9d7dc">
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<td class="line_content old" style="padding: inherit;" bgcolor="#fbe9eb"><pre style='display: block; font-size: 0.875rem; color: #333238; position: relative; font-family: var(--default-mono-font, "Menlo"),"DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'>-<span id="LC2501" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">double unifications is the main reason we disallow touchable</span></span>
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<td class="old_line diff-line-num old" data-linenumber="2502" style="width: 35px; color: rgba(31,30,36,0.24); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: inherit;" align="right" bgcolor="#f9d7dc">
2502
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<td class="new_line diff-line-num old" data-linenumber="2500" style="width: 35px; color: rgba(31,30,36,0.24); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: inherit;" align="right" bgcolor="#f9d7dc">
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<td class="line_content old" style="padding: inherit;" bgcolor="#fbe9eb"><pre style='display: block; font-size: 0.875rem; color: #333238; position: relative; font-family: var(--default-mono-font, "Menlo"),"DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'>-<span id="LC2502" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">unification variables as RHS of type family equations: F xis ~ alpha.</span></span>
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2503
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<td class="new_line diff-line-num old" data-linenumber="2500" style="width: 35px; color: rgba(31,30,36,0.24); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: inherit;" align="right" bgcolor="#f9d7dc">
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<td class="line_content old" style="padding: inherit;" bgcolor="#fbe9eb"><pre style='display: block; font-size: 0.875rem; color: #333238; position: relative; font-family: var(--default-mono-font, "Menlo"),"DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'>-<span id="LC2503" class="line" lang="haskell"></span>
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<td class="old_line diff-line-num" data-linenumber="2504" style="width: 35px; color: rgba(31,30,36,0.24); border-right-width: 1px; border-right-color: #ececef; border-right-style: solid; padding: inherit;" align="right" bgcolor="#fbfafd">
2504
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<td class="new_line diff-line-num" data-linenumber="2500" style="width: 35px; color: rgba(31,30,36,0.24); border-right-width: 1px; border-right-color: #ececef; border-right-style: solid; padding: inherit;" align="right" bgcolor="#fbfafd">
2500
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<td class="line_content" style="padding: inherit;"><pre style='display: block; font-size: 0.875rem; color: #333238; position: relative; font-family: var(--default-mono-font, "Menlo"),"DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'> <span id="LC2500" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Note [Do not unify representational equalities]</span></span>
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2505
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<td class="new_line diff-line-num" data-linenumber="2501" style="width: 35px; color: rgba(31,30,36,0.24); border-right-width: 1px; border-right-color: #ececef; border-right-style: solid; padding: inherit;" align="right" bgcolor="#fbfafd">
2501
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<td class="line_content" style="padding: inherit;"><pre style='display: block; font-size: 0.875rem; color: #333238; position: relative; font-family: var(--default-mono-font, "Menlo"),"DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'> <span id="LC2501" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~</span></span>
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<td class="old_line diff-line-num" data-linenumber="2506" style="width: 35px; color: rgba(31,30,36,0.24); border-right-width: 1px; border-right-color: #ececef; border-right-style: solid; padding: inherit;" align="right" bgcolor="#fbfafd">
2506
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<td class="new_line diff-line-num" data-linenumber="2502" style="width: 35px; color: rgba(31,30,36,0.24); border-right-width: 1px; border-right-color: #ececef; border-right-style: solid; padding: inherit;" align="right" bgcolor="#fbfafd">
2502
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<td class="line_content" style="padding: inherit;"><pre style='display: block; font-size: 0.875rem; color: #333238; position: relative; font-family: var(--default-mono-font, "Menlo"),"DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: inherit; border-radius: 2px; margin: 0; padding: 0; border: inherit solid #dcdcde;'> <span id="LC2502" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Consider [W] alpha ~R# b</span></span>
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