Haskell: correctness the compiler can prove

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.

Start the full course here: Haskell course: zero to hero in 50 lessons

A background in any strict language helps, see Development for the neighbouring courses.