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<h3>
Richard Eisenberg pushed to branch wip/T18300
at <a href="https://gitlab.haskell.org/ghc/ghc">Glasgow Haskell Compiler / GHC</a>
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Commits:
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<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/436819094bc1422044fd36c42384b1b1f977ae7d">43681909</a></strong>
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<span>by Richard Eisenberg</span>
<i>at 2020-06-16T17:56:55+01:00</i>
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<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Add Note [Do not always instantiate eagerly in types]
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<h4>1 changed file:</h4>
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<a href="#9355bef855426caf5f526925edf351b20f9a86c4">
compiler/GHC/Tc/Gen/HsType.hs
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<h4>Changes:</h4>
<li id="9355bef855426caf5f526925edf351b20f9a86c4">
<a href="https://gitlab.haskell.org/ghc/ghc/-/commit/436819094bc1422044fd36c42384b1b1f977ae7d#9355bef855426caf5f526925edf351b20f9a86c4"><strong>compiler/GHC/Tc/Gen/HsType.hs</strong></a>
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<td class="line_content match " style="padding-left: 0.5em; padding-right: 0.5em; color: rgba(0,0,0,0.3);" bgcolor="#fafafa">@@ -640,10 +640,12 @@ tcInferLHsTypeKind lhs_ty@(L loc hs_ty)</td>
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<pre style="margin: 0;"> <span id="LC640" class="line" lang="haskell"> <span class="n" style="color: #333;">setSrcSpan</span> <span class="n" style="color: #333;">loc</span> <span class="o" style="font-weight: 600;">$</span> <span class="c1" style="color: #998; font-style: italic;">-- Cover the tcInstInvisibleTyBinders</span></span>
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<pre style="margin: 0;"> <span id="LC641" class="line" lang="haskell"> <span class="kr" style="font-weight: 600;">do</span> <span class="p">{</span> <span class="p">(</span><span class="n" style="color: #333;">res_ty</span><span class="p">,</span> <span class="n" style="color: #333;">res_kind</span><span class="p">)</span> <span class="o" style="font-weight: 600;"><-</span> <span class="n" style="color: #333;">tc_infer_hs_type</span> <span class="p">(</span><span class="n" style="color: #333;">mkMode</span> <span class="kt" style="color: #458; font-weight: 600;">TypeLevel</span><span class="p">)</span> <span class="n" style="color: #333;">hs_ty</span></span>
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<pre style="margin: 0;"> <span id="LC642" class="line" lang="haskell"> <span class="p">;</span> <span class="n" style="color: #333;">tcInstInvisibleTyBinders</span> <span class="n" style="color: #333;">res_ty</span> <span class="n" style="color: #333;">res_kind</span> <span class="p">}</span></span>
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<pre style="margin: 0;">+<span id="LC643" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- See Note [Do not always instantiate eagerly in types]</span></span>
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<pre style="margin: 0;"> <span id="LC644" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC645" class="line" lang="haskell"><span class="c1" style="color: #998; font-style: italic;">-- Used to check the argument of GHCi :kind</span></span>
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<pre style="margin: 0;"> <span id="LC646" class="line" lang="haskell"><span class="c1" style="color: #998; font-style: italic;">-- Allow and report wildcards, e.g. :kind T _</span></span>
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<pre style="margin: 0;"> <span id="LC647" class="line" lang="haskell"><span class="c1" style="color: #998; font-style: italic;">-- Do not saturate family applications: see Note [Dealing with :kind]</span></span>
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<pre style="margin: 0;">+<span id="LC648" class="line" lang="haskell"><span class="c1" style="color: #998; font-style: italic;">-- Does not instantiate eagerly; See Note [Do not always instantiate eagerly in types]</span></span>
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<pre style="margin: 0;"> <span id="LC649" class="line" lang="haskell"><span class="n" style="color: #333;">tcInferLHsTypeUnsaturated</span> <span class="o" style="font-weight: 600;">::</span> <span class="kt" style="color: #458; font-weight: 600;">LHsType</span> <span class="kt" style="color: #458; font-weight: 600;">GhcRn</span> <span class="o" style="font-weight: 600;">-></span> <span class="kt" style="color: #458; font-weight: 600;">TcM</span> <span class="p">(</span><span class="kt" style="color: #458; font-weight: 600;">TcType</span><span class="p">,</span> <span class="kt" style="color: #458; font-weight: 600;">TcKind</span><span class="p">)</span></span>
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<pre style="margin: 0;"> <span id="LC650" class="line" lang="haskell"><span class="n" style="color: #333;">tcInferLHsTypeUnsaturated</span> <span class="n" style="color: #333;">hs_ty</span></span>
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<pre style="margin: 0;"> <span id="LC651" class="line" lang="haskell"> <span class="o" style="font-weight: 600;">=</span> <span class="n" style="color: #333;">addTypeCtxt</span> <span class="n" style="color: #333;">hs_ty</span> <span class="o" style="font-weight: 600;">$</span></span>
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<td class="line_content match " style="padding-left: 0.5em; padding-right: 0.5em; color: rgba(0,0,0,0.3);" bgcolor="#fafafa">@@ -677,6 +679,19 @@ to switch off saturation.</td>
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<pre style="margin: 0;"> <span id="LC679" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">So tcInferLHsTypeUnsaturated does a little special case for top level</span></span>
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<pre style="margin: 0;"> <span id="LC680" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">applications. Actually the common case is a bare variable, as above.</span></span>
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<pre style="margin: 0;"> <span id="LC681" class="line" lang="haskell"></span>
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<pre style="margin: 0;">+<span id="LC682" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Note [Do not always instantiate eagerly in types]</span></span>
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<pre style="margin: 0;">+<span id="LC683" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~</span></span>
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<pre style="margin: 0;">+<span id="LC684" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Terms are eagerly instantiated. This means that if you say</span></span>
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<pre style="margin: 0;">+<span id="LC685" class="line" lang="haskell"></span>
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<pre style="margin: 0;">+<span id="LC686" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> x = id</span></span>
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<pre style="margin: 0;">+<span id="LC687" class="line" lang="haskell"></span>
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<pre style="margin: 0;">+<span id="LC688" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">then `id` gets instantiated to have type alpha -> alpha. The variable</span></span>
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689
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<pre style="margin: 0;">+<span id="LC689" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">alpha is then unconstrained and regeneralized. But we cannot do this</span></span>
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690
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<pre style="margin: 0;">+<span id="LC690" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">in types, as we have no type-level lambda. So, when we are sure</span></span>
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691
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<pre style="margin: 0;">+<span id="LC691" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">that we will not want to regeneralize later -- because we are done</span></span>
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692
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<pre style="margin: 0;">+<span id="LC692" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">checking a type, for example -- we can instantiate. But we do not</span></span>
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693
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<pre style="margin: 0;">+<span id="LC693" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">instantiate at variables, nor do we in tcInferLHsTypeUnsaturated,</span></span>
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694
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<pre style="margin: 0;">+<span id="LC694" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">which is used by :kind in GHCi.</span></span>
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695
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<pre style="margin: 0;"> <span id="LC695" class="line" lang="haskell"></span>
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681
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696
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<pre style="margin: 0;"> <span id="LC696" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">************************************************************************</span></span>
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682
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697
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<pre style="margin: 0;"> <span id="LC697" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">* *</span></span>
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<td class="diff-line-num unfold js-unfold old_line" data-linenumber="1705" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">...</td>
<td class="diff-line-num unfold js-unfold new_line" data-linenumber="1720" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">...</td>
<td class="line_content match " style="padding-left: 0.5em; padding-right: 0.5em; color: rgba(0,0,0,0.3);" bgcolor="#fafafa">@@ -1705,6 +1720,7 @@ tc_lhs_pred mode pred = tc_lhs_type mode pred constraintKind</td>
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1705
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<pre style="margin: 0;"> <span id="LC1720" class="line" lang="haskell"><span class="n" style="color: #333;">tcTyVar</span> <span class="o" style="font-weight: 600;">::</span> <span class="kt" style="color: #458; font-weight: 600;">TcTyMode</span> <span class="o" style="font-weight: 600;">-></span> <span class="kt" style="color: #458; font-weight: 600;">Name</span> <span class="o" style="font-weight: 600;">-></span> <span class="kt" style="color: #458; font-weight: 600;">TcM</span> <span class="p">(</span><span class="kt" style="color: #458; font-weight: 600;">TcType</span><span class="p">,</span> <span class="kt" style="color: #458; font-weight: 600;">TcKind</span><span class="p">)</span></span>
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1706
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1721
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<pre style="margin: 0;"> <span id="LC1721" class="line" lang="haskell"><span class="c1" style="color: #998; font-style: italic;">-- See Note [Type checking recursive type and class declarations]</span></span>
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1707
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1722
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<pre style="margin: 0;"> <span id="LC1722" class="line" lang="haskell"><span class="c1" style="color: #998; font-style: italic;">-- in GHC.Tc.TyCl</span></span>
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1723
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<pre style="margin: 0;">+<span id="LC1723" class="line" lang="haskell"><span class="c1" style="color: #998; font-style: italic;">-- This does not instantiate. See Note [Do not always instantiate eagerly in types]</span></span>
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1708
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1724
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<pre style="margin: 0;"> <span id="LC1724" class="line" lang="haskell"><span class="n" style="color: #333;">tcTyVar</span> <span class="n" style="color: #333;">mode</span> <span class="n" style="color: #333;">name</span> <span class="c1" style="color: #998; font-style: italic;">-- Could be a tyvar, a tycon, or a datacon</span></span>
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1709
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1725
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<pre style="margin: 0;"> <span id="LC1725" class="line" lang="haskell"> <span class="o" style="font-weight: 600;">=</span> <span class="kr" style="font-weight: 600;">do</span> <span class="p">{</span> <span class="n" style="color: #333;">traceTc</span> <span class="s" style="color: #d14;">"lk1"</span> <span class="p">(</span><span class="n" style="color: #333;">ppr</span> <span class="n" style="color: #333;">name</span><span class="p">)</span></span>
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1710
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1726
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<pre style="margin: 0;"> <span id="LC1726" class="line" lang="haskell"> <span class="p">;</span> <span class="n" style="color: #333;">thing</span> <span class="o" style="font-weight: 600;"><-</span> <span class="n" style="color: #333;">tcLookup</span> <span class="n" style="color: #333;">name</span></span>
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