[karp-rabin] fix performance

src/Data/String/Search/KarpRabin.purs

@@ -192,12 +192,12 @@ strictMatcher { hash, hashMap, hLen, pats, rehash } = unsafePartial search
             go :: GoAcc -> Tuple Int (Array CodePoint) -> GoAcc
             go (Tuple h acc) (Tuple idx arr') =
               let rehashed = rehash h (fromJust $ A.head arr') (fromJust $ A.last arr')
-                  -- str' = S.fromCodePointArray arr'
-                  str' = S.drop idx str
-                  okay bs = isJust (S.stripPrefix (S.Pattern bs) str')
                   el = case M.lookup h hashMap of
                     Nothing -> Nothing
                     Just ps ->
+                      let str' = S.drop idx str
+                          okay bs = isJust (S.stripPrefix (S.Pattern bs) str')
+                      in
                       case A.filter (\x -> okay (A.unsafeIndex pats x)) ps of
                         [] -> Nothing
                         qs -> Just (Tuple idx qs)