Initial

.circleci/config.yml

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+version: 2.1
+
+jobs:
+  build-linux:
+    parameters:
+      version:
+        type: string
+    docker:
+      - image: haskell:<< parameters.version >>
+    steps:
+      - checkout
+      - restore_cache:
+          name: Restore
+          key: cache-<< parameters.version >>-{{ checksum "storable-offset.cabal" }}
+      - run:
+          name: Update Dependencies
+          command: |
+            cabal new-update && if [[ $(cabal --numeric-version) == '2.2.0.0' ]];
+                                  then cabal new-install
+                                  else cabal new-install --lib
+                                fi
+      - run:
+          name: Build
+          command: cabal new-build
+      - save_cache:
+          name: Cache
+          key: cache-<< parameters.version >>-{{ checksum "storable-offset.cabal" }}
+          paths:
+            - "/root/.cabal"
+            - "dist-newstyle"
+
+
+
+workflows:
+  workflow:
+    jobs:
+      - build-linux:
+          name: linux-8.2.2
+          version: 8.2.2
+      - build-linux:
+          name: linux-8.8.4
+          version: 8.8.4
+      - build-linux:
+          name: linux-9.2.2
+          version: 9.2.2-slim

LICENSE

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+Copyright Oleksii Divak (c) 2022
+
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+    * Redistributions of source code must retain the above copyright
+      notice, this list of conditions and the following disclaimer.
+
+    * 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.
+
+    * Neither the name of Author name here nor the names of other
+      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
+OWNER 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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+# storable-offset
+
+Tiny helper library that provides a way to access fields of `Storable` data structures.
+
+Given an instance
+
+```haskell
+{-# LANGUAGE DataKinds
+           , MultiParamTypeClasses #-}
+
+instance Offset "fieldName" DataType where rawOffset = #{offset data_type, field_name}
+```
+
+, you can now `peek`/`poke` `fieldName` directly using
+
+```haskell
+{-# LANGUAGE TypeApplications #-}
+
+peek $ offset @"fieldName" (ptr @DataType)
+
+poke (offset @"fieldName" ptr) (val @DataType)
+```

src/Foreign/Storable/Offset.hs

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+{-# LANGUAGE AllowAmbiguousTypes
+           , DataKinds
+           , KindSignatures
+           , MultiParamTypeClasses
+           , Rank2Types
+           , ScopedTypeVariables
+           , TypeApplications #-}
+
+{- | Utility functions for using 'Storable' to access record fields of data structures.
+  
+     Functions in this module rely on the [TypeApplications](https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/type_applications.html) pragma (the 'Offset' class itself doesn't).
+ -}
+
+module Foreign.Storable.Offset
+  ( -- * Offset
+    Offset (..)
+  , offset
+    -- * Helper functions
+  , pokeField
+  ) where
+
+import           Foreign.Ptr
+import           Foreign.Storable
+import           GHC.Records
+import           GHC.TypeLits
+
+
+
+class Offset (x :: Symbol) r where
+  -- | Byte distance between the start of datatype @r@ within memory and its field @x@.
+  rawOffset :: Int
+
+-- | Advances a pointer based on field's 'Offset', inheriting field's type.
+--
+--   Resulting pointer can be used to 'peek' or 'poke' the field directly.
+offset :: forall x r a. (HasField x r a, Offset x r) => Ptr r -> Ptr a
+offset = (`plusPtr` rawOffset @x @r)
+
+
+
+-- | Retrieves the field and 'poke's it at proper 'offset'.
+--   
+--   Useful for declaring 'poke' for the entire datatype.
+pokeField :: forall x r a. (HasField x r a, Offset x r, Storable a) => Ptr r -> r -> IO ()
+pokeField ptr = poke (offset @x @r ptr) . getField @x @r

storable-offset.cabal

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+cabal-version: 1.12
+
+name:           storable-offset
+version:        0.1.0.0
+description:    Please see the README on GitHub at <https://github.com/burningwitness/storable-offset#readme>
+homepage:       https://github.com/burningwitness/storable-offset#readme
+bug-reports:    https://github.com/burningwitness/storable-offset/issues
+author:         Oleksii Divak
+maintainer:     Oleksii Divak <frozenwitness@gmail.com>
+copyright:      2022 Oleksii Divak
+license:        BSD3
+license-file:   LICENSE
+category:       FFI, Utility
+synopsis:       Storable offsets for record fields
+build-type:     Simple
+extra-source-files:
+  README.md
+
+source-repository head
+  type: git
+  location: https://github.com/burningwitness/storable-offset
+
+library
+  exposed-modules:
+    Foreign.Storable.Offset
+  other-modules:
+    Paths_storable_offset
+  hs-source-dirs:
+    src
+  build-depends:
+    base >=4.10 && <4.17
+  default-language: Haskell2010
+  ghc-options: -Wall