[#45] Add newtype Id with phantom type parameter

Resolves #45

issue-wanted.cabal

@@ -70,7 +70,9 @@ library
                          IW.Config
 
                          -- Core Modules
+                         IW.Core.Id
                          IW.Core.Issue
+                         IW.Core.Repo
 
                          -- Database
                          IW.Db

src/IW/Core/Id.hs

@@ -0,0 +1,26 @@
+{-# OPTIONS_GHC -fno-warn-redundant-constraints #-}
+
+{-# LANGUAGE AllowAmbiguousTypes #-}
+
+module IW.Core.Id
+       ( Id (..)
+       , AnyId
+       , castId
+       ) where
+
+import Data.Type.Equality (type (==))
+
+
+-- | Wrapper for integer id. Contains phantom type parameter for increased
+-- type-safety.
+newtype Id a = Id { unId :: Int }
+    deriving stock (Show, Generic)
+    deriving newtype (Eq, Ord, FromField, ToField, FromJSON, ToJSON)
+
+-- | When we don't care about type of 'Id' but don't want to deal with type variables.
+type AnyId = Id ()
+
+-- | Unsafe cast of 'Id'. Implementation uses smart trick to enforce usage
+-- always with @TypeApplications@.
+castId :: forall to from to' . ((to == to') ~ 'True) => Id from -> Id to'
+castId (Id a) = Id a

src/IW/Core/Issue.hs

@@ -3,15 +3,19 @@
 module IW.Core.Issue
        ( Issue (..)
        ) where
+
+import IW.Core.Id (Id)
+import IW.Core.Repo (Repo)
+
 
 -- | Data type representing a GitHub issue.
 data Issue = Issue
-    { issueId     :: Int
+    { issueId     :: Id Issue
     , issueNumber :: Int
     , issueTitle  :: Text
     , issueBody   :: Text
     , issueUrl    :: Text
-    , issueRepoId :: Int
+    , issueRepoId :: Id Repo
     } deriving stock (Generic, Show, Eq)
       deriving anyclass (FromRow, ToRow)
       deriving (FromJSON, ToJSON) 

src/IW/Core/Repo.hs

@@ -0,0 +1,11 @@
+{-# LANGUAGE DeriveAnyClass #-}
+
+module IW.Core.Repo
+       ( Repo (..)
+       ) where
+
+import IW.Core.Id (Id)
+
+
+-- | Data type representing a GitHub repository.
+data Repo = Repo

src/IW/Db/Issue.hs

@@ -6,6 +6,7 @@ module IW.Db.Issue
        ( getIssues
        , getIssueById
        , getIssuesByLabel
+       , insertIssues
        ) where
 
 import IW.App (WithError)