Use strict Writer for P79

ninety-nine-haskell.cabal

@@ -44,7 +44,9 @@ library
       Paths_ninety_nine_haskell
   hs-source-dirs:
       src
-  ghc-options: -XTupleSections -Werror -Weverything -Wno-missing-import-lists -Wno-missed-specializations -Wno-all-missed-specializations -Wno-missing-local-signatures -Wno-monomorphism-restriction -Wno-missing-safe-haskell-mode -Wno-safe -Wno-unsafe -Wno-implicit-prelude -Wno-prepositive-qualified-module -Wno-missing-kind-signatures -Wno-unused-top-binds -Wno-missing-export-lists
+  default-extensions:
+      TupleSections
+  ghc-options: -Werror -Weverything -Wno-missing-import-lists -Wno-missed-specializations -Wno-all-missed-specializations -Wno-missing-local-signatures -Wno-monomorphism-restriction -Wno-missing-safe-haskell-mode -Wno-safe -Wno-unsafe -Wno-implicit-prelude -Wno-prepositive-qualified-module -Wno-missing-kind-signatures -Wno-unused-top-binds -Wno-missing-export-lists
   build-depends:
       array
     , base >=4.7 && <5
@@ -82,7 +84,9 @@ test-suite ninety-nine-test
       Paths_ninety_nine_haskell
   hs-source-dirs:
       test
-  ghc-options: -XTupleSections -Werror -Weverything -Wno-missing-import-lists -Wno-missed-specializations -Wno-all-missed-specializations -Wno-missing-local-signatures -Wno-monomorphism-restriction -Wno-missing-safe-haskell-mode -Wno-safe -Wno-unsafe -Wno-implicit-prelude -Wno-prepositive-qualified-module -Wno-missing-kind-signatures -Wno-unused-top-binds -Wno-missing-export-lists
+  default-extensions:
+      TupleSections
+  ghc-options: -Werror -Weverything -Wno-missing-import-lists -Wno-missed-specializations -Wno-all-missed-specializations -Wno-missing-local-signatures -Wno-monomorphism-restriction -Wno-missing-safe-haskell-mode -Wno-safe -Wno-unsafe -Wno-implicit-prelude -Wno-prepositive-qualified-module -Wno-missing-kind-signatures -Wno-unused-top-binds -Wno-missing-export-lists
   build-depends:
       QuickCheck
     , base >=4.7 && <5

package.yaml

@@ -25,8 +25,10 @@ dependencies:
 - split
 - containers
 
+default-extensions:
+- TupleSections
+
 ghc-options:
-- -XTupleSections
 - -Werror
 # For details on warnings: https://downloads.haskell.org/~ghc/master/users-guide/using-warnings.html
 # This list originally taken from https://medium.com/mercury-bank/enable-all-the-warnings-a0517bc081c3

src/Monads.hs

@@ -1,4 +1,5 @@
 {-# LANGUAGE DerivingStrategies #-}
+{-# LANGUAGE FlexibleContexts #-}
 
 module Monads
   ( Operator (..),
@@ -10,12 +11,12 @@ module Monads
 where
 
 import qualified Control.Monad as M
-import Control.Monad.State (State)
+import Control.Monad.State (MonadState, State)
 import qualified Control.Monad.State as S
 import Control.Monad.Trans.Maybe (MaybeT)
 import qualified Control.Monad.Trans.Maybe as Mb
-import Control.Monad.Writer (Writer, WriterT)
-import qualified Control.Monad.Writer as W
+import Control.Monad.Writer.Strict (MonadWriter, Writer, WriterT)
+import qualified Control.Monad.Writer.Strict as W
 import Data.Monoid (Sum (..))
 import qualified Data.Monoid as Md
 
@@ -159,6 +160,12 @@ type Result = Maybe Integer
 Stack order matters. The output is in the reverse order,
 i.e. the innermost monad result wraps the others.
 ((a, w), s), where a is Result, w is Logs, and s is Stack.
+
+This implementation uses the "tagless final" approach:
+- Functions are declared using type constraints instead of
+  concrete types.
+- Instantiation of a polymorphic function to a concrete type
+  (aka interpreter) happens "at the end":
 -}
 type Calculation = MaybeT (WriterT Logs (State Stack)) Integer
 
@@ -167,14 +174,22 @@ calculatePostfix = fst . chain . calc
   where
     chain = flip S.runState [] . W.runWriterT . Mb.runMaybeT
 
-calc :: [Element] -> Calculation
+calc ::
+  (MonadWriter Logs m, MonadState Stack m, MonadFail m) =>
+  [Element] ->
+  m Integer
 calc elems =
   S.get >>= case elems of
     [] -> result
     (Operand n : xs) -> loop xs Nothing . (n :)
     (Operator op : xs) -> runOp op M.>=> loop xs (Just op)
 
-loop :: [Element] -> Maybe Operator -> Stack -> Calculation
+loop ::
+  (MonadWriter Logs m, MonadState Stack m, MonadFail m) =>
+  [Element] ->
+  Maybe Operator ->
+  Stack ->
+  m Integer
 loop xs op s = W.tell [(s, op)] >> S.put s >> calc xs
 
 {-