Online Haskell Compiler

Haskell is a statically-typed, purely-functional programming language with non-strict (lazy) evaluation. The reference compiler GHC produces fast native binaries despite the language forbidding mutation in the source — purity lets the runtime parallelise and memoise aggressively. Haskell is the canonical language for studying type theory, monads, and lazy evaluation.

GHC takes five-to-ten minutes to install the first time, and Stack or Cabal add their own learning curves on top. A browser sandbox removes both barriers for coursework, interview preparation (Haskell questions appear at firms like Galois and Standard Chartered), or pure curiosity about how a monad actually works.

Compiler construction (Alex + Happy), domain-specific languages (the EDSLs in Pandoc, Shake, and Cryptol), financial modelling libraries at firms like Standard Chartered, formal verification research, parallel code via the par combinator, and parser combinators (Parsec, megaparsec). The sandbox runs GHC.

Paste a snippet — main = putStrLn "hi, Tutorsbot" — hit Run, see stdout. The highlighter handles Haskell's layout rule (indentation-driven syntax), list syntax [a], tuple syntax (a, b), type signatures (:: String -> String), and do-notation for monadic sequencing.

An online Haskell compiler runs real GHC underneath, so type classes, monadic do-notation and the layout rule all behave exactly as they would locally — including the type errors, which show up at compile time rather than as a runtime surprise. Paste a signature or a full module, hit Run, and GHC's actual diagnostics appear in the output panel within seconds; sessions autosave and every run gets a link worth sharing when you're debugging a monad transformer stack with someone else. Treat it as a free Haskell interpreter for checking whether a type even compiles, or as a low-friction way to run Haskell code in the browser while learning how laziness actually behaves.

Comparing typed functional styles? The Online Scala Compiler is a natural companion on the JVM side, and the Online Rust Compiler is worth a look if you're weighing Haskell's purity against Rust's ownership model.

Installing GHC the first time is a five-to-ten-minute affair even before Stack or Cabal enter the picture, which is a lot of setup just to check whether a type signature unifies. Skip it: paste or write Haskell code in the editor, press Run or Ctrl+Enter, and read GHC's actual output below — the same diagnostics a local ghc invocation would produce, not an approximation. It's the fastest way to see why a Functor instance won't typecheck, or to confirm a lazy-evaluation trick really does what the textbook claims.

If the rest of your stack is numerical, the Online Julia Compiler is one click away, and the full catalogue of languages lives on the Online Compiler hub.

GHC has a well-earned reputation for being heavy to install — the toolchain, Stack or Cabal, and the first full package build can take real time and real disk space. This free online Haskell compiler gives you the compiler's genuine diagnostics with none of that: paste code, run it, read GHC's output, done. A local install is still what you want once you're pulling in real dependencies from Hackage, but for checking a type class instance or a parser combinator snippet, the sandbox is faster.

The Online Julia Compiler is a good companion for numerical work, and the rest of the catalogue is listed on the free online compiler hub.