For solving the AoC2016 advent calendar. http://adventofcode.com/2016

Project structure inspired by (Tomash Aschan)[https://github.com/tomasaschan/advent-of-code-2018].

Instructions

Running this in 2025, I installed Stack using GHCUp https://www.haskell.org/ghcup/, since that worked well.

Code structure and running solvers

One package per day called dayXX. Each package has their own executables, tests etc. Run with stack run dayXX or stack test dayXX.

There is also a package util for reused code.

Developing aid

Formatters

Use ormolu. It is opinionated. Command line call it using below. There is a VSCode extension for it, but I did not get it to work with the multi-cradle and stack setup, so I just used the CLI directly.

stack exec ormolu -- --mode inplace  some/file/path.hs 

Profiling Haskell

This is well developed, but good tutorials are hard to come by. I've seen this relevant reading:

• Video tutorial with GHC https://www.youtube.com/watch?v=pwcEUdf4Qmk
• Stack tool profiling options. https://docs.haskellstack.org/en/stable/GUIDE/#debugging
• Pasing other options to GHC when calling Stack https://docs.haskellstack.org/en/stable/GUIDE/#ghc-options
• GHC own profiling https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/profiling.html
• Profiling via Criterion and Stack. There is No full tutorial, bu combine https://stackoverflow.com/questions/37485522/how-to-use-criterion-with-stack with http://www.serpentine.com/criterion/tutorial.html to get somewhere.

Time profiling

stack build --executable-profiling
stack exec --profile -- aoc2016 <dayNr> +RTS -p -hc
cat aoc2016.EXE.prof

Space profiling

Assumes that you dont have a ps-viewer, but you have installed the ps2pdf program instead. :)

stack build --executable-profiling
stack exec --profile -- aoc2016 <dayNr> +RTS -hc
stack exec hp2ps -- aoc2016.EXE
ps2pdf aoc2016.EXE.ps
open aoc2016.EXE.pdf

If you don't have ps2pdf, you can get SVG output with stack install hp2pretty and then

stack build --executable-profiling;
stack exec --profile -- aoc2016 <dayNr> +RTS -hc; 
stack exec hp2pretty -- aoc2016.EXE.hp; open aoc2016.EXE.svg

This is a cost centre based report. change the -hc to -hT or -hy to get varous report types.

Code coverage

The standard setup described in https://docs.haskellstack.org/en/stable/coverage/#code-coverage is not suitable for this package, since the library is compiled into the executable. Instead, we must use GHC coverage options directly.

stack build --ghc-options -fhpc # both builds the main binary so that it emits coverage data when run
aoc2016 <dayNr1> # run whatever days you like
aoc2016 <dayNr2> # run whatever days you like
stack exec hpc -- markup aoc2016.EXE.tix # compile the report from the days you ran
open hpc_index.html # look at the report

To get coverage for the tests, I have only managed a strange workaround. The Test runner is a temporary script Main.hs. So we can get the report for the test script. But not its libraries. Very strange.

stack test --ghc-options -fhpc
open Main.hs.html

Documentation

Some Modules (e.g. Day 14) script have documentation in them. Compile them with Haddock. Read on that page for all options. https://haskell-haddock.readthedocs.io/en/latest/index.html

stack install haddock
stack exec haddock -- --html  old/src/Day14.hs
open Day14.html

Debugging

the GHCi debugger is quite nice. Run stack ghci, then load modules of choice, set breakpoints with :break, show available variables with :show bindings etc.

See The documentation on GHCi debugging

Trace debugging can be helpful, see Debug.Trace at https://hackage.haskell.org/package/base-4.16.0.0/docs/Debug-Trace.html#v:trace

Example:

import Debug.Trace
fib :: Integer -> Integer
fib 0 = trace "0 => 0" $ 0
fib 1 = trace "1 => 1" $ 1
fib n =
   let input = n
       output = fib (n-1) + fib (n-2)
   in trace (show input ++ " => " ++ show output) $ output

IDE support

I have been using VSCode with the haskell language server. It is generally good, can find, parse and present haddock snippets, autoformat with ormolu or stylish-haskell and more. There is one problem though. By default, the build system (stack in my case) is supposed to explain to hie-bios how various parts of the project fits together. When there are several Main modules in the stack project, this mechanism fails. This can be fixed, by manually configuring a multi-cradle project so that each project package is handeled with its own Main-module. You need to write the name of a build target there, so call stack ide build-target to list them. That is a way to debug the package.yaml files as well.

Therefore, the hie.yaml must be carried around all the time. It is a bummer, but it is acceptable, I guess.

If you get the error

Multi Cradle: No prefixes matched
pwd: C:\Users\Ludvig\Google Drive\Hobbyprojekt 2021\adventofcode2016
filepath: C:\Users\Ludvig\Google Drive\Hobbyprojekt 2021\adventofcode2016\day18\app\Main.hs
prefixes:
("./old",Stack {component = Just "old:exe:old", stackYaml = Nothing})
("./day19",Stack {component = Just "day19:exe:day19", stackYaml = Nothing})
("./day20",Stack {component = Just "day20:exe:day20", stackYaml = Nothing})
("./day21",Stack {component = Just "day21:exe:day21", stackYaml = Nothing})

or similar, it means that the file you have opened does not match any of the file path prefixes (e.g. folders) in the multi-cradle setup. Maybe you added a new package, without adding a hie cradle configuration?