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<TITLE>weird behaviour of context resolution with FlexibleContexts and TypeFamilies</TITLE>
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<P><FONT SIZE=2 FACE="Courier New">Hi,</FONT>
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<P><FONT SIZE=2 FACE="Courier New">If I build the code below with -DVER=2, I get a complaint about PatchInspect (PrimOf p) being missing from the context of cleverNamedResolve.</FONT></P>
<P><FONT SIZE=2 FACE="Courier New">This doesn't happen with -DVER=1 or -DVER=3</FONT>
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<P><FONT SIZE=2 FACE="Courier New">I presume that type class resolution is operating slightly differently in the different cases, but it's quite confusing - in the original code joinPatches did something useful and I was trying to inline the known instance definition. I would have expected it to be consistent between all three cases, either requiring the context or not.</FONT></P>
<P><FONT SIZE=2 FACE="Courier New">Is it a bug, or just one of the risks one takes by using FlexibleContexts?</FONT>
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<P><FONT SIZE=2 FACE="Courier New">I've tried this with GHC 6.12.3 and with 7.0.2RC2.</FONT>
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<P><FONT SIZE=2 FACE="Courier New">Cheers,</FONT>
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<P><FONT SIZE=2 FACE="Courier New">Ganesh</FONT>
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<P><FONT SIZE=2 FACE="Courier New">{-# LANGUAGE CPP, TypeFamilies, FlexibleContexts #-}</FONT>
<BR><FONT SIZE=2 FACE="Courier New">module Class ( cleverNamedResolve ) where</FONT>
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<P><FONT SIZE=2 FACE="Courier New">data FL p = FL p</FONT>
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<P><FONT SIZE=2 FACE="Courier New">class PatchInspect p where</FONT>
<BR><FONT SIZE=2 FACE="Courier New">instance PatchInspect p => PatchInspect (FL p) where</FONT>
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<P><FONT SIZE=2 FACE="Courier New">type family PrimOf p</FONT>
<BR><FONT SIZE=2 FACE="Courier New">type instance PrimOf (FL p) = PrimOf p</FONT>
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<P><FONT SIZE=2 FACE="Courier New">data WithName prim = WithName prim</FONT>
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<P><FONT SIZE=2 FACE="Courier New">instance PatchInspect prim => PatchInspect (WithName prim) where</FONT>
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<P><FONT SIZE=2 FACE="Courier New">class (PatchInspect (PrimOf p)) => Conflict p where</FONT>
<BR><FONT SIZE=2 FACE="Courier New"> resolveConflicts :: p -> PrimOf p</FONT>
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<P><FONT SIZE=2 FACE="Courier New">instance Conflict p => Conflict (FL p) where</FONT>
<BR><FONT SIZE=2 FACE="Courier New"> resolveConflicts = undefined</FONT>
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<P><FONT SIZE=2 FACE="Courier New">type family OnPrim p</FONT>
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<P><FONT SIZE=2 FACE="Courier New">#if VER==1</FONT>
<BR><FONT SIZE=2 FACE="Courier New">class FromPrims p where</FONT>
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<P><FONT SIZE=2 FACE="Courier New">instance FromPrims (FL p) where</FONT>
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<P><FONT SIZE=2 FACE="Courier New">joinPatches :: FromPrims p => p -> p</FONT>
<BR><FONT SIZE=2 FACE="Courier New">#else</FONT>
<BR><FONT SIZE=2 FACE="Courier New">#if VER==2</FONT>
<BR><FONT SIZE=2 FACE="Courier New">joinPatches :: FL p -> FL p</FONT>
<BR><FONT SIZE=2 FACE="Courier New">#else</FONT>
<BR><FONT SIZE=2 FACE="Courier New">joinPatches :: p -> p</FONT>
<BR><FONT SIZE=2 FACE="Courier New">#endif</FONT>
<BR><FONT SIZE=2 FACE="Courier New">#endif</FONT>
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<P><FONT SIZE=2 FACE="Courier New">joinPatches = id</FONT>
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<P><FONT SIZE=2 FACE="Courier New">cleverNamedResolve :: (Conflict (OnPrim p)</FONT>
<BR><FONT SIZE=2 FACE="Courier New"> ,PrimOf (OnPrim p) ~ WithName (PrimOf p))</FONT>
<BR><FONT SIZE=2 FACE="Courier New"> => FL (OnPrim p) -> WithName (PrimOf p)</FONT>
<BR><FONT SIZE=2 FACE="Courier New">cleverNamedResolve = resolveConflicts . joinPatches</FONT>
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