GHCi: the interactive shell

Start and quit

GHCi reads one expression, evaluates it, prints the result, and waits. That loop is the fastest way to answer a question about the language without creating a file.

ghci

GHCi, version 9.10.1: https://www.haskell.org/ghc/ :? for help ghci> 2 + 2 4 ghci> :q Leaving GHCi.

A prompt starting with ghci> means the module Prelude is in scope, so the standard functions are available without an import.

Load a file

Save a definition to Main.hs, then load it with :l. The prompt changes to *Main>, which means the module is loaded with its internal names visible.

square :: Int -> Int square x = x * x

ghci Main.hs

ghci> square 12 144 ghci> :reload Ok, one module loaded.

:reload re-reads the file after an edit, so the editor and the prompt can stay open side by side.

Ask about types

:t prints the type of an expression without evaluating it. When a type error confuses you, ask for the type of the argument and the type of the function separately.

ghci> :t square square :: Int -> Int ghci> :t map map :: (a -> b) -> [a] -> [b] ghci> :t 42 42 :: Num a => a

The third answer says the literal works for any numeric type. GHCi defaults it to Integer when it prints a value, but the type stays general until something pins it down.

Browse classes

:i shows where a name comes from, its signature, and every instance of the class it belongs to. It answers which types can be compared faster than any documentation page.

ghci> :i Ord type Ord :: * -> Constraint class Eq a => Ord a where compare :: a -> a -> Ordering ... ghci> :i Bool data Bool = False | True

:? lists every command, and :browse Data.List prints the exports of a module you have imported.

Settings

Two flags make the prompt more useful. Multiline input lets you write a definition over several lines, and a shorter prompt leaves room for long expressions.

ghci -XMultiLineInput

ghci> :set prompt "> " > let twice f x = f (f x) > twice (+1) 5 7

Multiline input ends with an empty line, and a definition written this way lives only in the session. Anything you want to keep belongs in a file loaded with :l.

Next: Your first program

Article author: Arthur Isaev