Overview
This section is a walkthrough of a project with the code intended to display how to write a simple but real-world application in Haskell.
The subsections are ordered by difficulty.
Tip
We strongly encourage reading Haskell code in a text editor with the IDE activated: mousing over variables will show their type and where they are defined, which is especially helpful when learning. Cmd + click to jump to a definition.
Demo:¶
cabal run chess-repl
Welcome!
>
place a white bishop on a5
_|_|_|_|b|_|_|_
_|_|_|_|_|_|_|_
_|_|_|_|_|_|_|_
_|_|_|_|_|_|_|_
_|_|_|_|_|_|_|_
_|_|_|_|_|_|_|_
_|_|_|_|_|_|_|_
_|_|_|_|_|_|_|_
>
place a black castle on b6
1:15:
|
1 | place a black castle on b6
| ^^^^^^
unexpected "castle"
expecting "bishop", "king", "knight", "pawn", "queen", "rook", or white space
>
:d
_|_|_|_|b|_|_|_
_|_|_|_|_|_|_|_
_|_|_|_|_|_|_|_
_|_|_|_|_|_|_|_
_|_|_|_|_|_|_|_
_|_|_|_|_|_|_|_
_|_|_|_|_|_|_|_
_|_|_|_|_|_|_|_
>
:q
Tip
If you encounter an expression in this codebase that you don't understand, use Haskell's purity and immutability to your advantage: you can't always break it down into its parts and understand them separately.
main :: IO ()
main = runInputT defaultSettings $ flip evalStateT initBoard $ forever $ do
line <- lift requestLine
...
Break this down by replacing everything after the first or second dollar sign with undefined, and then mouse over undefined to understand its type:
Or
Or remove everything before the first or second dollar sign and have Haskell infer the type:
-- main :: IO () -- commented out since the type might be different now
main = flip evalStateT initBoard $ forever $ do
line <- lift requestLine
...
Consider this the Haskell debugging analogue of inserting print statements.
Created: January 8, 2023