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However, it is very much considered bad style not to write type signatures for top-level definitions. Values of type IO () describe actions which can interact with the outside world.īecause Haskell has a fully-fledged Hindley-Milner type system which allows for automatic type inference, type signatures are technically optional: if you simply omit the main :: IO (), the compiler will be able to infer the type on its own by analyzing the definition of main. This first line is a type signature, declaring the type of main: main :: IO () Compiling Main ( helloworld.hs, interpreted ) :reload (or :r) reloads everything in ghci: Prelude> :l helloworld.hs
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It comes shipped with most Haskell environments, such as ghci which comes with the GHC compiler: ghci> putStrLn "Hello World!"Īlternatively, load scripts into ghci from a file using load (or :l): ghci> :load helloworld The interactive REPL can also be used instead of compiling. helloworldĪlternatively, runhaskell or runghc make it possible to run the program in interpreted mode without having to compile it: runhaskell helloworld.hs Put this into a helloworld.hs file and compile it using a Haskell compiler, such as GHC: ghc helloworld.hsĮxecuting the compiled file will result in the output "Hello, World!" being printed to the screen. This type annotation is usually omitted for main because it is its only possible type. The first line is an optional type annotation, indicating that main is a value of type IO (), representing an I/O action which "computes" a value of type () (read "unit" the empty tuple conveying no information) besides performing some side effects on the outside world (here, printing a string at the terminal). Common functors as the base of cofree comonadsĪ basic "Hello, World!" program in Haskell can be expressed concisely in just one or two lines: main :: IO ().Arbitrary-rank polymorphism with RankNTypes.