# Installing GHC, Cabal, Stack and GHCi — Haskell

Source: https://www.geekswithgeeks.com/en/haskell/f-setup

> Install the toolchain with GHCup and work in GHCi and Cabal projects.

## The Haskell toolchain

The recommended installer is **GHCup**, which installs and switches between versions of **GHC** (the compiler), **cabal-install** (the build tool and package manager), **Stack** (an alternative build tool using curated package sets) and **HLS** (the Haskell Language Server, which powers editor features in VS Code and others). **GHCi** is the interactive REPL: evaluate expressions, `:t expr` shows a type, `:i name` shows information about a type or class, `:l File.hs` loads a file and `:r` reloads it. Libraries are published on **Hackage**; **Stackage** provides snapshots of package versions known to build together. A **Cabal project** has a `.cabal` file describing the package, its library, executables and test suites, their `build-depends` with version bounds, and the `default-language` (such as `GHC2021`, a bundle of widely used extensions). Common commands: `cabal init`, `cabal build`, `cabal run`, `cabal repl` and `cabal test`. Searching by type signature with **Hoogle** is a Haskell superpower: searching `[a] -> Int` finds `length`.

## Creating and exploring a project

GHCup, a Cabal package and GHCi commands.

```bash
ghcup tui                      # interactive installer: choose GHC, cabal, HLS versions
ghc --version
cabal --version

mkdir shop && cd shop
cabal init --non-interactive --exe   # creates shop.cabal and app/Main.hs
cabal run                       # build and run
cabal repl                      # GHCi with the project loaded

# shop.cabal (excerpt)
# executable shop
#     main-is:          Main.hs
#     hs-source-dirs:   app
#     build-depends:    base >=4.17 && <5, containers, text
#     default-language: GHC2021
#     ghc-options:      -Wall

# Inside GHCi:
#   ghci> :t map
#   map :: (a -> b) -> [a] -> [b]
#   ghci> :i Maybe
#   ghci> :r              -- reload after editing
```

## A workshop with a catalogue

GHCup sets up the workshop, Cabal builds your projects, Hackage is the parts catalogue, and Hoogle is a catalogue you can search by the shape of the part you need rather than its name.

**Quiz:** What does the GHCi command :t do?

- [ ] Runs tests
- [ ] Terminates GHCi
- [ ] Toggles laziness
- [x] Shows the type of an expression

*Answer:* Shows the type of an expression. :t (or :type) prints the inferred type of an expression.