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author | Sebastian Graf <sebastian.graf@kit.edu> | 2020-02-20 16:48:16 +0100 |
---|---|---|
committer | Sebastian Graf <sebastian.graf@kit.edu> | 2020-02-26 13:35:54 +0100 |
commit | 054e7ac57418c55f1004cf432cb3a5485464fd4b (patch) | |
tree | f03d566d38a9a52e479f5b2d8a22fc4bc8390089 /compiler/GHC/HsToCore/Expr.hs | |
parent | 9d09411122b9b534b96e988b6d3f6d7eb04b8f66 (diff) | |
download | haskell-wip/pmcheck-ldi.tar.gz |
PmCheck: Implement Long-distance information with Covered setswip/pmcheck-ldi
Consider
```hs
data T = A | B | C
f :: T -> Int
f A = 1
f x = case x of
A -> 2
B -> 3
C -> 4
```
Clearly, the RHS returning 2 is redundant. But we don't currently see
that, because our approximation to the covered set of the inner case
expression just picks up the positive information from surrounding
pattern matches. It lacks the context sensivity that `x` can't be `A`
anymore!
Therefore, we adopt the conceptually and practically superior approach
of reusing the covered set of a particular GRHS from an outer pattern
match. In this case, we begin checking the `case` expression with the
covered set of `f`s second clause, which encodes the information that
`x` can't be `A` anymore. After this MR, we will successfully warn about
the RHS returning 2 being redundant.
Perhaps surprisingly, this was a great simplification to the code of
both the coverage checker and the desugarer.
Found a redundant case alternative in `unix` submodule, so we have to
bump it with a fix.
Metric Decrease:
T12227
Diffstat (limited to 'compiler/GHC/HsToCore/Expr.hs')
-rw-r--r-- | compiler/GHC/HsToCore/Expr.hs | 11 |
1 files changed, 6 insertions, 5 deletions
diff --git a/compiler/GHC/HsToCore/Expr.hs b/compiler/GHC/HsToCore/Expr.hs index b627d6e841..6328d473ec 100644 --- a/compiler/GHC/HsToCore/Expr.hs +++ b/compiler/GHC/HsToCore/Expr.hs @@ -69,6 +69,7 @@ import Outputable import PatSyn import Control.Monad +import Data.List.NonEmpty ( nonEmpty ) {- ************************************************************************ @@ -216,8 +217,8 @@ dsUnliftedBind (PatBind {pat_lhs = pat, pat_rhs = grhss , pat_ext = NPatBindTc _ ty }) body = -- let C x# y# = rhs in body -- ==> case rhs of C x# y# -> body - do { rhs <- dsGuarded grhss ty - ; checkGuardMatches PatBindGuards grhss + do { rhs_deltas <- checkGuardMatches PatBindGuards grhss + ; rhs <- dsGuarded grhss ty (nonEmpty rhs_deltas) ; let upat = unLoc pat eqn = EqnInfo { eqn_pats = [upat], eqn_orig = FromSource, @@ -446,9 +447,9 @@ dsExpr (HsMultiIf res_ty alts) = mkErrorExpr | otherwise - = do { match_result <- liftM (foldr1 combineMatchResults) - (mapM (dsGRHS IfAlt res_ty) alts) - ; checkGuardMatches IfAlt (GRHSs noExtField alts (noLoc emptyLocalBinds)) + = do { let grhss = GRHSs noExtField alts (noLoc emptyLocalBinds) + ; rhss_deltas <- checkGuardMatches IfAlt grhss + ; match_result <- dsGRHSs IfAlt grhss res_ty (nonEmpty rhss_deltas) ; error_expr <- mkErrorExpr ; extractMatchResult match_result error_expr } where |