diff options
Diffstat (limited to 'compiler/GHC/Rename/Bind.hs')
| -rw-r--r-- | compiler/GHC/Rename/Bind.hs | 57 |
1 files changed, 43 insertions, 14 deletions
diff --git a/compiler/GHC/Rename/Bind.hs b/compiler/GHC/Rename/Bind.hs index 21789ef89a..7f3edf841c 100644 --- a/compiler/GHC/Rename/Bind.hs +++ b/compiler/GHC/Rename/Bind.hs @@ -493,18 +493,10 @@ rnBind _ bind@(PatBind { pat_lhs = pat bind' = bind { pat_rhs = grhss' , pat_ext = fvs' } - ok_nobind_pat - = -- See Note [Pattern bindings that bind no variables] - case unLoc pat of - WildPat {} -> True - BangPat {} -> True -- #9127, #13646 - SplicePat {} -> True - _ -> False - -- Warn if the pattern binds no variables -- See Note [Pattern bindings that bind no variables] ; whenWOptM Opt_WarnUnusedPatternBinds $ - when (null bndrs && not ok_nobind_pat) $ + when (null bndrs && not (isOkNoBindPattern pat)) $ addTcRnDiagnostic (TcRnUnusedPatternBinds bind') ; fvs' `seq` -- See Note [Free-variable space leak] @@ -540,29 +532,66 @@ rnBind sig_fn (PatSynBind x bind) rnBind _ b = pprPanic "rnBind" (ppr b) + -- See Note [Pattern bindings that bind no variables] +isOkNoBindPattern :: LPat GhcRn -> Bool +isOkNoBindPattern (L _ pat) = + case pat of + WildPat{} -> True -- Exception (1) + BangPat {} -> True -- Exception (2) #9127, #13646 + p -> patternContainsSplice p -- Exception (3) + + where + lpatternContainsSplice :: LPat GhcRn -> Bool + lpatternContainsSplice (L _ p) = patternContainsSplice p + patternContainsSplice :: Pat GhcRn -> Bool + patternContainsSplice p = + case p of + -- A top-level splice has been evaluated by this point, so we know the pattern it is evaluated to + SplicePat (HsUntypedSpliceTop _ p) _ -> patternContainsSplice p + -- A nested splice isn't evaluated so we can't guess what it will expand to + SplicePat (HsUntypedSpliceNested {}) _ -> True + -- The base cases + VarPat {} -> False + WildPat {} -> False + LitPat {} -> False + NPat {} -> False + NPlusKPat {} -> False + -- Recursive cases + BangPat _ lp -> lpatternContainsSplice lp + LazyPat _ lp -> lpatternContainsSplice lp + AsPat _ _ _ lp -> lpatternContainsSplice lp + ParPat _ _ lp _ -> lpatternContainsSplice lp + ViewPat _ _ lp -> lpatternContainsSplice lp + SigPat _ lp _ -> lpatternContainsSplice lp + ListPat _ lps -> any lpatternContainsSplice lps + TuplePat _ lps _ -> any lpatternContainsSplice lps + SumPat _ lp _ _ -> lpatternContainsSplice lp + ConPat _ _ cpd -> any lpatternContainsSplice (hsConPatArgs cpd) + XPat (HsPatExpanded _orig new) -> patternContainsSplice new + {- Note [Pattern bindings that bind no variables] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Generally, we want to warn about pattern bindings like Just _ = e because they don't do anything! But we have three exceptions: -* A wildcard pattern +(1) A wildcard pattern _ = rhs which (a) is not that different from _v = rhs (b) is sometimes used to give a type sig for, or an occurrence of, a variable on the RHS -* A strict pattern binding; that is, one with an outermost bang +(2) A strict pattern binding; that is, one with an outermost bang !Just _ = e This can fail, so unlike the lazy variant, it is not a no-op. Moreover, #13646 argues that even for single constructor types, you might want to write the constructor. See also #9127. -* A splice pattern +(3) A splice pattern $(th-lhs) = rhs It is impossible to determine whether or not th-lhs really - binds any variable. We should disable the warning for any pattern - which contain splices, but that is a more expensive check. + binds any variable. You have to recurse all the way into the pattern to check + it doesn't contain any splices like this. See #22057. Note [Free-variable space leak] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |