Haskell basics: what the language is and where it is used

What Haskell is

Haskell is a purely functional language with a type system that catches whole classes of bugs before the program starts. A function cannot quietly reach into the outside world, a missing case in a pattern match is a warning, and a value that may be absent has a type that says so.

The language dates from 1990 and shaped what later languages borrow: type classes, immutable data, lazy evaluation, and the do notation that hides sequencing behind a readable block. There is no null: a missing value is a Maybe, and the compiler makes you handle it.

Where it is used

Haskell is strong where correctness matters more than a quick first draft: compilers and language tools, financial modelling, data pipelines, configuration and network services. Cardano and many industrial trading systems are written in it, and GHC itself is written in Haskell.

A background in any strict language helps, see Development for the neighbouring courses. The language is a good second language precisely because it makes different choices: no mutation, no statements, and functions as ordinary values.

Hello, world

A Haskell program is a set of definitions, and the runtime calls main. Put this in a file Hello.hs:

main :: IO () main = putStrLn "Hello, world!"

Run it with the interpreter, which needs no build step:

runghc Hello.hs

Hello, world!

The same file compiles to a native binary:

ghc Hello.hs

[1 of 1] Compiling Main ( Hello.hs, Hello.o ) Linking hello ...

IO () is the type of an action: it may talk to the world and returns nothing useful. A function without IO in its signature cannot print, read a clock, or change a global variable, which is what makes the rest of the program easy to test.

The course

The full course is fifty lessons, from this first program to a working command line tool: Haskell course: zero to hero in 50 lessons. Every page ends with a link to the next one.

Article author: Arthur Isaev