Chris Stryczynski

Software Developer / Consultant

Haskell Tutorial - Command line to do application

Posted on: 20/02/2018

Section 0 - Introduction

Right, so this is a first tutorial / guide / technical article I’m writing with Hart - This means you will be able to clone down this project and follow along (and hack) at each section - seriously just do this:

git clone

Within each section there will be two commits, the ‘from’ commit which is BEFORE the changes have occured relating to that section, and the until commit which is AFTER. So after cloning down the repo you can do a git reset --HARD commitIdBEFORE and follow along with the changes, or just git reset --HARD commitIdAFTER to see the end result of that section.

Feedback is always appreciated.

Right so what are we building here? We’ll be building a simple command line based, todo list manager.

Section 1 - First steps

Git From Commit: 

Git Until Commit: 

Right so we’ll be storing the values as a JSON file. No we won’t be using Hadoop, a database or some crazy quantum encryption device - let’s be pragmatic.

First we need to add the time library to cabal:

diff --git a/app.cabal b/app.cabal
index 4def916..1abd6ea 100644
--- a/app.cabal
+++ b/app.cabal
@@ -20,6 +20,7 @@ executable app
   -- other-modules:       
   -- other-extensions:    
   build-depends:       base >=4.10 && <4.11
+                     , time
   -- hs-source-dirs:      
   default-language:    Haskell2010
   hs-source-dirs: src

Lets define the types we know so far, and some imports.

Section 2 - Input output

Git From Commit: 

Git Until Commit: 

Right lets add some functions relating to the functionality! We’ll just mock this for now.

Some potentially interesting things here: undefined is essentially a function that returns an error, it’s often used as a ‘placeholder’ to later on add an actual implementation. In Haskell you can’t just have an ‘empty’ function - it has to return a value. The _ as the function parameter for saveTodos means we are ignoring this value (as we only plan to use this value later on).

Section 3 - Jsonify

Git From Commit: 

Git Until Commit: 

Right, time to actually make this work!

Right lets add some functions relating to the functionality! We’ll just mock this for now.

First we need some additional libraries, so we add them to cabal:

We add JSON encoding / decoding instance for our Todo data type, functionality to actually save and retrieve the files, and lastly we need add the ability to process a ‘save’ command.

diff --git a/src/Main.hs b/src/Main.hs
index c42e702..d833564 100644
--- a/src/Main.hs
+++ b/src/Main.hs
@@ -1,23 +1,40 @@
+{-# LANGUAGE DeriveGeneric #-}
 module Main where
 import Data.Time.Calendar
 import System.Environment
+import Data.Aeson
+import GHC.Generics
+import Data.String.Conversions
+import Data.Time.Format
+import System.Directory
 type TodoTitle = String
-data Todo = Todo TodoTitle Day
+data Todo = Todo TodoTitle Day deriving (Generic)
+instance FromJSON Todo
+instance ToJSON Todo
 instance Show Todo where
   show (Todo tt d) = show d ++ " - " ++ tt
+todoSavePath :: FilePath
+todoSavePath = "todoData.json"
 saveTodos :: [Todo] -> IO ()
-saveTodos _ = undefined
+saveTodos t = do
+  writeFile todoSavePath (convertString $ encode t)
 todos :: IO [Todo]
-todos = return $ [
-    Todo "Finish this tutorial" ( fromGregorian 2017 01 16 )
-  , Todo "Talk dog for walk" ( fromGregorian 2017 01 16)
-  , Todo "Sleep" ( fromGregorian 2017 01 16)
-  ]
+todos = do
+  fileExists <- doesFileExist todoSavePath
+  if fileExists then do
+    jsonEncoded <- readFile todoSavePath
+    case (decode $ convertString jsonEncoded :: Maybe [Todo]) of
+      Just x -> return x
+      Nothing -> error "Failed parsing JSON"
+  else return []
 main :: IO ()
 main = do
@@ -25,6 +42,13 @@ main = do
   args <- getArgs
   case args of
     ([]) -> todos >>= mapM_ print
+    ("save":[]) -> error "Not able to parse correct amount of parameters"
+    ("save":title:due:[]) -> do
+      case (parseTimeM False defaultTimeLocale "%x" due :: Maybe Day) of
+        Just day -> do
+          let newTodo = Todo title day
+          existingTodos <- todos
+          saveTodos (newTodo : existingTodos)
+          putStrLn $ "The following todo has been saved: \n" ++ show newTodo
+        Nothing -> error "Failed parsing due date"
     _ -> error "Not able to parse correct amount of parameters"

Add now you should be able to:

stack exec app -- save "Testing todo" 01/01/18

And see output of:

The following todo has been saved: 
2018-01-01 - Testing todo

And to list all the todos:

stack exec app

And see output of:

2018-01-01 - Testing todo

Great! We’ve got the core functionality implemented!


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