Free monads sample project

Only builds for now with the DSL fleshed out.

Signed-off-by: Sanchayan Maity <maitysanchayan@gmail.com>

.gitignore

@@ -0,0 +1,3 @@
+free-monad-example.cabal
+.stack-work/
+*~

LICENSE

@@ -0,0 +1,30 @@
+Copyright Sanchayan Maity (c) 2020
+
+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

@@ -0,0 +1 @@
+# free-monad-example

Setup.hs

@@ -0,0 +1,2 @@
+import Distribution.Simple
+main = defaultMain

app/Main.hs

@@ -0,0 +1,6 @@
+module Main where
+
+import Lib
+
+main :: IO ()
+main = putStrLn "Free monad example"

hie.yaml

@@ -0,0 +1,13 @@
+cradle:
+  stack:
+    - path: "./src"
+      component: "free-monad-example:lib"
+
+    - path: "./app/Main.hs"
+      component: "free-monad-example:exe:free-monad-example-exe"
+
+    - path: "./app/Paths_free_monad_example.hs"
+      component: "free-monad-example:exe:free-monad-example-exe"
+
+    - path: "./test"
+      component: "free-monad-example:test:free-monad-example-test"

package.yaml

@@ -0,0 +1,56 @@
+name:                free-monad-example
+version:             0.1.0.0
+github:              "sanchayanmaity/free-monad-example"
+license:             BSD3
+author:              "Sanchayan Maity"
+maintainer:          "maitysanchayan@gmail.com"
+copyright:           "2020 Sanchayan Maity"
+
+extra-source-files:
+- README.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/sanchayanmaity/free-monad-example#readme>
+
+dependencies:
+- base >= 4.7 && < 5
+- free
+- postgresql-simple
+- servant
+- servant-server
+- text
+
+ghc-options:
+- -Wall
+- -Wunused-binds
+
+library:
+  source-dirs: src
+
+executables:
+  free-monad-example-exe:
+    main:                Main.hs
+    source-dirs:         app
+    ghc-options:
+    - -threaded
+    - -rtsopts
+    - -with-rtsopts=-N
+    dependencies:
+    - free-monad-example
+
+tests:
+  free-monad-example-test:
+    main:                Spec.hs
+    source-dirs:         test
+    ghc-options:
+    - -threaded
+    - -rtsopts
+    - -with-rtsopts=-N
+    dependencies:
+    - free-monad-example

src/Lib.hs

@@ -0,0 +1,97 @@
+{-# LANGUAGE GADTs #-}
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE Rank2Types #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+
+module Lib where
+
+import Control.Exception
+import Control.Monad.Free.Church
+import Database.PostgreSQL.Simple
+import Data.Proxy
+import Data.Text
+import Servant.API
+import Servant.Server
+
+data SqlDBMethodF next where
+  SqlDBMethod :: FromRow r => (Connection -> Query -> IO [r]) -> ([r] -> next) -> SqlDBMethodF next
+  SqlThrowException :: Exception e => e -> (a -> next) -> SqlDBMethodF next
+
+instance Functor SqlDBMethodF where
+  fmap f (SqlDBMethod runner next) = SqlDBMethod runner (f . next)
+  fmap f (SqlThrowException exceptionMsg next) = SqlThrowException exceptionMsg (f . next)
+
+type SqlDb = F SqlDBMethodF
+
+sqlDbMethod :: FromRow r => (Connection -> Query -> IO [r]) -> SqlDb [r]
+sqlDbMethod action = liftF $ SqlDBMethod action id
+
+sqlThrowException :: Exception e => e -> SqlDb a
+sqlThrowException exception = liftF $ SqlThrowException exception id
+
+interpretSqlDBMethod :: Connection -> Query -> SqlDBMethodF a -> IO a
+interpretSqlDBMethod conn squery (SqlDBMethod runner next) = next <$> runner conn squery
+interpretSqlDBMethod _ _ (SqlThrowException exception _) = throwIO exception
+
+runDB :: Connection -> Query -> SqlDb a -> IO a
+runDB conn squery = foldF $ interpretSqlDBMethod conn squery
+
+type SampleAPI = Get '[JSON] Text
+
+data ServeF next where
+  ServeMethod :: (forall api. HasServer api '[] => (Proxy api -> IO a)) -> (a -> next) -> ServeF next
+
+instance Functor ServeF where
+  fmap f (ServeMethod runner next) = ServeMethod runner (f . next)
+
+type Serve = F ServeF
+
+serveMethod :: (forall api. HasServer api '[] => (Proxy api -> IO a)) -> Serve a
+serveMethod action = liftF $ ServeMethod action id
+
+interpretServeMethod :: HasServer api '[] => Proxy api -> ServeF a -> IO a
+interpretServeMethod api (ServeMethod runner next) = next <$> runner api
+
+runServe :: HasServer api '[] => Proxy api -> Serve a -> IO a
+runServe api = foldF $ interpretServeMethod api
+
+data LoggerMethodF next where
+  LogMessage :: Text -> (() -> next) -> LoggerMethodF next
+
+instance Functor LoggerMethodF where
+  fmap f (LogMessage msg next) = LogMessage msg (f . next)
+
+type Logger = F LoggerMethodF
+
+logMessage :: Text -> Logger ()
+logMessage logMsg = liftF $ LogMessage logMsg id
+
+interpretLoggerMethod :: LoggerMethodF a -> IO a
+interpretLoggerMethod (LogMessage msg next) = next <$> print msg
+
+runLogger :: Logger a -> IO a
+runLogger = foldF interpretLoggerMethod
+
+data DSLMethod next where
+  LogMsg :: Logger () -> (() -> next) -> DSLMethod next
+  ServeHttp :: HasServer api '[] => Proxy api -> Serve a -> (a -> next) -> DSLMethod next
+  RunQuery :: FromRow r => Connection -> Query -> SqlDb [r] -> ([r] -> next) -> DSLMethod next
+
+instance Functor DSLMethod where
+  fmap f (LogMsg msg next) = LogMsg msg (f . next)
+  fmap f (ServeHttp api act next) = ServeHttp api act (f . next)
+  fmap f (RunQuery conn squery act next) = RunQuery conn squery act (f . next)
+
+type DSL = F DSLMethod
+
+interpretDSLMethod :: DSLMethod a -> IO a
+interpretDSLMethod (LogMsg logger next) =
+  fmap next $ runLogger logger
+interpretDSLMethod (ServeHttp api server next) =
+  fmap next $ runServe api server
+interpretDSLMethod (RunQuery conn squery sql next) =
+  fmap next $ runDB conn squery sql
+
+runDSL :: DSL a -> IO a
+runDSL = foldF interpretDSLMethod

stack.yaml

@@ -0,0 +1,66 @@
+# 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/yaml_configuration/
+
+# Resolver to choose 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:
+#
+# resolver: lts-3.5
+# resolver: nightly-2015-09-21
+# resolver: ghc-7.10.2
+#
+# 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.
+#
+# resolver: ./custom-snapshot.yaml
+# resolver: https://example.com/snapshots/2018-01-01.yaml
+resolver: lts-15.15
+
+# 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 resolver.
+# 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 local 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: ">=2.3"
+#
+# 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

@@ -0,0 +1,12 @@
+# 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/lock_files
+
+packages: []
+snapshots:
+- completed:
+    size: 496112
+    url: https://raw.githubusercontent.com/commercialhaskell/stackage-snapshots/master/lts/15/15.yaml
+    sha256: 86169722ad0056ffc9eacc157ef80ee21d7024f92c0d2961c89ccf432db230a3
+  original: lts-15.15

test/Spec.hs

@@ -0,0 +1,2 @@
+main :: IO ()
+main = putStrLn "Test suite not yet implemented"