Call-by-push-block

Call-by-push-block is a sokoban game written in 10 lines of Haskell for the Feb '23 Haskell Tiny Game Jam. It is for the Prelude category and so does not use any imports.

In Call-by-push-block, you are code golfing. To clear a level, you must move the lambda (λ) to push a block (o) into every hole (_).

Your score is the number of moves you take. Like in real golf, a lower score is better, but make sure you can complete the level first before you try to get the best score.

There are 15 levels of roughly increasing difficulty. They will take around an hour to complete, depending on experience. Your scores for each level are given at the end: compete with your friends to see who can get the lowest scores!

General advice

• You need to press Enter to submit your move (a quirk of this entry being in the Prelude category).
• Try everything! You can always undo or reset if you reach an unsolvable state.
• Read the Controls in detail section if you're confused by the controls.
• Read the "Hints" section if you get stuck.

Quick start

I have Haskell installed

Run this for the vanilla user experience (stack runghc works too).

$ runghc call-by-push-block.hs

Run this version if you do not like having to press Enter after every move. Note that this version is provided for convenience and does not fit the rules and category of the game jam.

$ runghc call-by-push-block-no-buffering.hs

I don't want to install Haskell

All credit for this solution goes to Radon and Radian for creating and hosting Riju.

First, open up either call-by-push-block.hs or call-by-push-block-no-buffering.hs and copy the source code.

Then, go to https://riju.codes/haskell.

Paste the code and hit Run.

You'll be able to play the game in the terminal on the right side.

More details

See the Running section for more details.

The cast

• λ: The player character.
• o: A block you can push.
• _: The hole you need to push a block into.
• Joined by several others!

Scoring

• Your score is displayed beneath the level number, starting at 1.
• It increments for every move and undo you make and resets whenever you reset the level.
• A lower score is better, except for a score of 0 which indicates an incompleted level.

Controls in detail

Note that you need to press Enter in order to make a move.

• Movement: wasd
  • w: up
  • s: down
  • a: left
  • d: right
• Reset: x
  • Resets the level. Also resets your score.
• Undo: u
  • Undoes one move. This feature is added for convenience since moves must be sent using the Enter key (and thus it takes longer to get back where you were if you make a mistake) and incurs a small score penalty.
• Additional notes
  • When prompted with ↩, press Enter to continue.
  • Technically, all keys map to a control. Try to avoid pressing an unlabeled key as you can accidentally skip a level this way.

Hints

*WARNING: Do not read this in plaintext, as the hints are "hidden" using <details> tags. Read the comments in call-by-push-block.hs for rot-13 enciphered hints.

I recommend revealing them in order until you find one that helps. If you are completely stuck on a level, there is a level skip code at the end.

• Levels 1-5
  • Spoiler

    You can direct the player character off the grid. This will require you to reset in order to beat the level.
  • Spoiler

    The hash is a static obstacle. Like a hole, it cannot be moved or passed through. You also cannot push blocks into it.
  • Spoiler

    You can push multiple blocks at a time.
  • Spoiler

    You can push blocks off of the grid. Be careful not to push a block you need off the grid.
  • Spoiler

    You need an empty tile adjacent to a block in order to push it. For example, to push a block down, it needs an empty tile above it. Keep this in mind even for later levels since it will allow you to identify holes that are inaccessible to certain blocks.
• Levels 5-8
  • Spoiler

    The first hint is no longer true: there are conditions where pushing a player character off the grid is necessary. Remember: you only need to push a block into each hole.
  • Spoiler

    The order you try things in often matters. Sometimes you may have the right idea but wrong execution order.
  • Spoiler

    Reset or undo early if you realize you are in an unwinnable state. A quick trick to determine whether you are in one is to count the number of remaining holes and check if there is a block that can be pushed into each. Refer also to the last hint of the previous section.
• Levels 9-15
  • Spoiler

    The plus is a tile that destroys both player characters and blocks alike. When a block is pushed into it or a player character moves into it, both the plus and the object encountering it are replaced with an empty tile.
  • Spoiler

    Take advantage of hashes or holes to move some player characters will keeping others stationary.
  • Spoiler

    Elaborating on the above hint, you will find that some things which seem impossible are only so because your player characters are not coordinated in space properly. Sometimes you need to coordinate them by moving one and keeping another stationary. Other times they will be separated in an unwinnable state.
• Level skip cheat code
  • Spoiler

    Type "Cheat Code" (case sensitive) and press enter to skip the level. This will confer a score of 0, indicating the level was skipped. If you are playing on the unbuffered version, just typing the first letter will suffice.

Running

First, make sure that you have Haskell installed. I recommend using GHCUp. This game is tested on GHC 9.2.5 but should run on most modern GHC versions.

I highly recommend running call-by-push-block-no-buffering.hs for a better experience, although it might be behind call-by-push-block.hs if I make any more changes since I probably won't update the two at the same time.

Both files require no additional arguments and can be run using runghc. For example,

$ runghc call-by-push-block.hs

Or with stack,

$ stack runghc call-by-push-block.hs

File structure

• call-by-push-block.hs is the submission to the game jam.
• call-by-push-block-no-buffering.hs is a version of call-by-push-block.hs with unbuffered IO which means that you do not have to hit Enter after each move.
• call-by-push-block-ungolfed.hs is a much earlier version that I wrote before golfing it. It has, however, diverged significantly from the golfed version.
• all-levels.txt is all of the levels in order and their encoded form. This contains spoilers on the level contents.
• puzzle-scratch.txt is where I have sketches of the levels (including some alternate/initial versions and unused levels). This contains spoilers on the level contents and solutions.
• encode-levels contains code to compress the levels into Unicode characters. This folder may spoilers on level content (particularly encode-levels/levels-raw.txt).

Acknowledgements

• Golfing: I've learned a bunch about code golfing in Haskell from various tips and threads I've read from inumerably many people. I got some explicit help from The Ninteenth Byte on Stack Exchange with shaving characters off. Thank you!
• Testing and design: I solicited help from a bunch of people who graciously lent me their time and feedback. Thank you!
• The jam and feedback: #haskell-game and the organizers of The Haskell Tiny Game Jam. Thank you!
• Playing the game in the browser: Radon and Radian for creating and hosting Riju.