- = EqInst
- { tci_left = mkTyConApp fam args
- , tci_right = rwi_right eq
- , tci_co = rwi_co eq
- , tci_loc = rwi_loc eq
- , tci_name = rwi_name eq
- }
+ = deriveEqInst eq (mkTyConApp fam args) (rwi_right eq) (rwi_co eq)
+
+-- Derive an EqInst based from a RewriteInst, possibly swapping the types
+-- around.
+--
+deriveEqInst :: RewriteInst -> TcType -> TcType -> EqInstCo -> TcM Inst
+deriveEqInst rewrite ty1 ty2 co
+ = do { co_adjusted <- if not swapped then return co
+ else mkSymEqInstCo co (ty2, ty1)
+ ; return $ EqInst
+ { tci_left = left
+ , tci_right = right
+ , tci_co = co_adjusted
+ , tci_loc = rwi_loc rewrite
+ , tci_name = rwi_name rewrite
+ }
+ }
+ where
+ swapped = rwi_swapped rewrite
+ (left, right) = if not swapped then (ty1, ty2) else (ty2, ty1)
+
+instance Outputable RewriteInst where
+ ppr (RewriteFam {rwi_fam = fam, rwi_args = args, rwi_right = rhs, rwi_co =co})
+ = hsep [ pprEqInstCo co <+> text "::"
+ , ppr (mkTyConApp fam args)
+ , text "~>"
+ , ppr rhs
+ ]
+ ppr (RewriteVar {rwi_var = tv, rwi_right = rhs, rwi_co =co})
+ = hsep [ pprEqInstCo co <+> text "::"
+ , ppr tv
+ , text "~>"
+ , ppr rhs
+ ]
+
+pprEqInstCo :: EqInstCo -> SDoc
+pprEqInstCo (Left cotv) = ptext (sLit "Wanted") <+> ppr cotv
+pprEqInstCo (Right co) = ptext (sLit "Local") <+> ppr co