oops, i already implemented an applicative

.gitignore

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+.stack-work/
+*~

CHANGELOG.md

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+# Changelog for `java-classfile`
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
+and this project adheres to the
+[Haskell Package Versioning Policy](https://pvp.haskell.org/).
+
+## Unreleased
+
+## 0.1.0.0 - YYYY-MM-DD

LICENSE

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+Copyright 2025 Author name here
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+1.  Redistributions of source code must retain the above copyright notice, this
+    list of conditions and the following disclaimer.
+
+2.  Redistributions in binary form must reproduce the above copyright notice,
+    this list of conditions and the following disclaimer in the documentation
+    and/or other materials provided with the distribution.
+
+3.  Neither the name of the copyright holder nor the names of its contributors
+    may be used to endorse or promote products derived from this software
+    without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR
+ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
+ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

README.md

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+# java-classfile

Setup.hs

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+import Distribution.Simple
+main = defaultMain

app/Main.hs

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+module Main (main) where
+
+main :: IO ()
+main = pure ()

java-classfile.cabal

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+cabal-version: 2.2
+
+-- This file has been generated from package.yaml by hpack version 0.38.1.
+--
+-- see: https://github.com/sol/hpack
+
+name:           java-classfile
+version:        0.1.0.0
+description:    Please see the README on GitHub at <https://github.com/githubuser/java-classfile#readme>
+homepage:       https://github.com/githubuser/java-classfile#readme
+bug-reports:    https://github.com/githubuser/java-classfile/issues
+author:         Author name here
+maintainer:     example@example.com
+copyright:      2025 Author name here
+license:        BSD-3-Clause
+license-file:   LICENSE
+build-type:     Simple
+extra-source-files:
+    README.md
+    CHANGELOG.md
+
+source-repository head
+  type: git
+  location: https://github.com/githubuser/java-classfile
+
+library
+  exposed-modules:
+      Language.Java.Classfile
+      Language.Java.Classfile.Extract
+      Language.Java.Classfile.Extractable
+      Language.Java.Classfile.Version
+  other-modules:
+      Paths_java_classfile
+  autogen-modules:
+      Paths_java_classfile
+  hs-source-dirs:
+      src
+  ghc-options: -Wall -Wcompat -Widentities -Wincomplete-record-updates -Wincomplete-uni-patterns -Wmissing-export-lists -Wmissing-home-modules -Wpartial-fields -Wredundant-constraints
+  build-depends:
+      base >=4.7 && <5
+    , bytestring
+  default-language: Haskell2010
+
+executable java-classfile-exe
+  main-is: Main.hs
+  other-modules:
+      Paths_java_classfile
+  autogen-modules:
+      Paths_java_classfile
+  hs-source-dirs:
+      app
+  ghc-options: -Wall -Wcompat -Widentities -Wincomplete-record-updates -Wincomplete-uni-patterns -Wmissing-export-lists -Wmissing-home-modules -Wpartial-fields -Wredundant-constraints -threaded -rtsopts -with-rtsopts=-N
+  build-depends:
+      base >=4.7 && <5
+    , bytestring
+    , java-classfile
+  default-language: Haskell2010
+
+test-suite java-classfile-test
+  type: exitcode-stdio-1.0
+  main-is: Spec.hs
+  other-modules:
+      Paths_java_classfile
+  autogen-modules:
+      Paths_java_classfile
+  hs-source-dirs:
+      test
+  ghc-options: -Wall -Wcompat -Widentities -Wincomplete-record-updates -Wincomplete-uni-patterns -Wmissing-export-lists -Wmissing-home-modules -Wpartial-fields -Wredundant-constraints -threaded -rtsopts -with-rtsopts=-N
+  build-depends:
+      base >=4.7 && <5
+    , bytestring
+    , java-classfile
+  default-language: Haskell2010

package.yaml

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+name:                java-classfile
+version:             0.1.0.0
+github:              "githubuser/java-classfile"
+license:             BSD-3-Clause
+author:              "Author name here"
+maintainer:          "example@example.com"
+copyright:           "2025 Author name here"
+
+extra-source-files:
+- README.md
+- CHANGELOG.md
+
+# Metadata used when publishing your package
+# synopsis:            Short description of your package
+# category:            Web
+
+# To avoid duplicated efforts in documentation and dealing with the
+# complications of embedding Haddock markup inside cabal files, it is
+# common to point users to the README.md file.
+description:         Please see the README on GitHub at <https://github.com/githubuser/java-classfile#readme>
+
+dependencies:
+- base >= 4.7 && < 5
+- bytestring
+
+ghc-options:
+- -Wall
+- -Wcompat
+- -Widentities
+- -Wincomplete-record-updates
+- -Wincomplete-uni-patterns
+- -Wmissing-export-lists
+- -Wmissing-home-modules
+- -Wpartial-fields
+- -Wredundant-constraints
+
+library:
+  source-dirs: src
+
+executables:
+  java-classfile-exe:
+    main:                Main.hs
+    source-dirs:         app
+    ghc-options:
+    - -threaded
+    - -rtsopts
+    - -with-rtsopts=-N
+    dependencies:
+    - java-classfile
+
+tests:
+  java-classfile-test:
+    main:                Spec.hs
+    source-dirs:         test
+    ghc-options:
+    - -threaded
+    - -rtsopts
+    - -with-rtsopts=-N
+    dependencies:
+    - java-classfile

src/Language/Java/Classfile.hs

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+module Language.Java.Classfile () where
+import Language.Java.Classfile.Version (Version)
+
+data Classfile = Classfile
+  { version :: Version
+  }

src/Language/Java/Classfile/Extract.hs

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+{-# LANGUAGE DeriveFunctor #-}
+{-# LANGUAGE InstanceSigs #-}
+module Language.Java.Classfile.Extract (Extract()) where
+
+import Data.ByteString.Lazy (ByteString)
+
+data Extract a = Extract (Continuation a)
+
+type Continuation a = ByteString -> Reply a
+
+data Reply a
+  = Done ByteString a -- rest, result
+  | Fail
+  deriving (Functor)
+
+instance Functor Extract where
+  fmap :: (a -> b) -> Extract a -> Extract b
+  fmap f (Extract cont) = Extract $ \ input -> f <$> cont input
+
+instance Applicative Extract where
+  pure :: a -> Extract a
+  pure x = Extract $ \ rest -> Done rest x
+  (<*>) :: Extract (a -> b) -> Extract a -> Extract b
+  (<*>) (Extract computeF) (Extract computeX) = Extract $ \ input -> case computeF input of
+    Done rest f -> case computeX rest of
+      Done rest' x -> Done rest' (f x)
+      Fail -> Fail
+    Fail -> Fail
+
+
+
+{- It seems I cannot define a lawful monad instance
+instance Monad Extract where
+  (>>=) :: Extract a -> (a -> Extract b) -> Extract b
+  (>>=) (Extract computeA) f = Extract $ \ input -> case computeA input of
+    Done rest a -> _
+    Fail -> Fail
+-}
+

src/Language/Java/Classfile/Extractable.hs

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+module M () where
+
+
+class Extractable a where
+  extract :: Extract a
+
+instance Extractable Word8 where
+  extract :: Extract Word8
+  extract = _

src/Language/Java/Classfile/Version.hs

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+module Language.Java.Classfile.Version (Version(..)) where
+
+import Data.Word (Word16)
+
+data Version = Version
+  { minor :: Word16
+  , major :: Word16
+  }

stack.yaml

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+# This file was automatically generated by 'stack init'
+#
+# Some commonly used options have been documented as comments in this file.
+# For advanced use and comprehensive documentation of the format, please see:
+# https://docs.haskellstack.org/en/stable/configure/yaml/
+
+# A 'specific' Stackage snapshot or a compiler version.
+# A snapshot resolver dictates the compiler version and the set of packages
+# to be used for project dependencies. For example:
+#
+# snapshot: lts-23.24
+# snapshot: nightly-2025-06-15
+# snapshot: ghc-9.8.4
+#
+# The location of a snapshot can be provided as a file or url. Stack assumes
+# a snapshot provided as a file might change, whereas a url resource does not.
+#
+# snapshot: ./custom-snapshot.yaml
+# snapshot: https://example.com/snapshots/2024-01-01.yaml
+snapshot: nightly-2025-07-10
+
+# User packages to be built.
+# Various formats can be used as shown in the example below.
+#
+# packages:
+# - some-directory
+# - https://example.com/foo/bar/baz-0.0.2.tar.gz
+#   subdirs:
+#   - auto-update
+#   - wai
+packages:
+- .
+# Dependency packages to be pulled from upstream that are not in the snapshot.
+# These entries can reference officially published versions as well as
+# forks / in-progress versions pinned to a git hash. For example:
+#
+# extra-deps:
+# - acme-missiles-0.3
+# - git: https://github.com/commercialhaskell/stack.git
+#   commit: e7b331f14bcffb8367cd58fbfc8b40ec7642100a
+#
+# extra-deps: []
+
+# Override default flag values for project packages and extra-deps
+# flags: {}
+
+# Extra package databases containing global packages
+# extra-package-dbs: []
+
+# Control whether we use the GHC we find on the path
+# system-ghc: true
+#
+# Require a specific version of Stack, using version ranges
+# require-stack-version: -any # Default
+# require-stack-version: ">=3.7"
+#
+# Override the architecture used by Stack, especially useful on Windows
+# arch: i386
+# arch: x86_64
+#
+# Extra directories used by Stack for building
+# extra-include-dirs: [/path/to/dir]
+# extra-lib-dirs: [/path/to/dir]
+#
+# Allow a newer minor version of GHC than the snapshot specifies
+# compiler-check: newer-minor

stack.yaml.lock

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+# This file was autogenerated by Stack.
+# You should not edit this file by hand.
+# For more information, please see the documentation at:
+#   https://docs.haskellstack.org/en/stable/topics/lock_files
+
+packages: []
+snapshots:
+- completed:
+    sha256: 39e3a4dc79edf153bb0aa6dc4206b1543c0e0f3e3811ec751abdcdf3aaf08887
+    size: 723871
+    url: https://raw.githubusercontent.com/commercialhaskell/stackage-snapshots/master/nightly/2025/7/10.yaml
+  original: nightly-2025-07-10

test/Spec.hs

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+main :: IO ()
+main = putStrLn "Test suite not yet implemented"