# Performance, Profiling and Tooling — Haskell

Source: https://www.geekswithgeeks.com/en/haskell/x-performance

> Build optimised programs, profile them and keep code consistent.

## Making Haskell fast and maintainable

GHC is an optimising compiler: build releases with **`-O2`** (Cabal uses `-O1` by default), and use `-threaded` with `+RTS -N` for multicore programs. Haskell can be very fast, but performance needs attention to **data representation and strictness**: use `Text`, `ByteString`, `Vector` and strict maps instead of `String` and lists for large data; add strict fields and strict accumulators; avoid `nub` (quadratic) in favour of `Set` or `Map`; and beware of space leaks from laziness. **Profile** before optimising: build with profiling (`cabal build --enable-profiling`) and run with `+RTS -p` for a time and allocation report, `+RTS -s` for garbage collector statistics, and heap profiles (`-hT` or `-hc`) viewed with **eventlog2html** to find leaks. **Criterion** or **tasty-bench** benchmark functions reliably. For code quality: **HLint** suggests simplifications, **Ormolu** or **Fourmolu** format code, `-Wall -Werror` in CI catches warnings, and **HLS** in the editor shows types, errors and refactorings as you type. **Haddock** generates documentation from comments.

## Profiling and quality commands

Optimised builds, RTS statistics, profiling and linting.

```bash
# optimised, multicore build
cabal build --ghc-options="-O2 -threaded -rtsopts"
cabal run shop -- +RTS -N -s          # -N: all cores, -s: GC and memory summary

# time and allocation profile
cabal run shop --enable-profiling -- +RTS -p    # writes shop.prof

# heap profile by closure type, rendered as HTML
cabal run shop --enable-profiling -- +RTS -hT -l
eventlog2html shop.eventlog

# benchmarks (tasty-bench or criterion suite in the .cabal file)
cabal bench

# code quality
hlint src app test                    # suggestions such as "use concatMap"
fourmolu --mode inplace $(git ls-files '*.hs')
cabal build --ghc-options="-Wall -Werror"
cabal haddock                         # generate API documentation
```

## A doctor's check-up before surgery

Profiling is the check-up: it tells you which organ actually hurts. Optimising without it is surgery on a guess, often on code that was never the problem.

**Quiz:** Which RTS flag prints a garbage collection and memory usage summary?

- [ ] +RTS -N
- [x] +RTS -s
- [ ] +RTS -p
- [ ] -O2

*Answer:* +RTS -s. +RTS -s prints allocation, GC and memory statistics at exit.