.hs File Extension

What kind of file is a .hs?

A .hs file contains ordinary Haskell source code. It is editable text, not a compiled executable. A file commonly defines one module with declarations for functions, types, classes, and values. GHC recognizes .hs as Haskell source that should be preprocessed and compiled.¹

There are no fixed magic bytes. Editors and build tools identify the language from the suffix, project context, or source syntax. Haskell source uses Unicode characters, and UTF-8 is the practical encoding for current projects.² IANA does not register a Haskell-specific media type, although some software uses the unregistered text/x-haskell. Poly classifies .hs as readable text/plain.³

How do I open a .hs on my computer?

Any text editor can open .hs files on Windows, macOS, or Linux. Visual Studio Code with Haskell Language Server, IntelliJ IDEA with a Haskell plugin, Vim, Neovim, and Emacs can add syntax highlighting, completion, navigation, and diagnostics.

Install GHC when you need to check, run, or compile the code. GHCi can load a source file interactively, while GHC can build an executable:⁴

ghci Main.hs
ghc Main.hs -o my-program

For a real project, follow its Cabal or Stack instructions so dependencies and compiler options match. Opening source as text does not run it. Compiling, evaluating, or running unfamiliar code can execute arbitrary behavior and download dependencies, so review it first.

Poly support for .hs

Poly opens .hs files in a code-friendly viewer with Haskell syntax highlighting, word wrap, and in-file search. It indexes readable source for exact and semantic search, so you can find a symbol by name or search for code by purpose.

The Poly agent can summarize a module, explain a type signature, and help trace likely behavior. Poly does not compile the file, evaluate expressions, resolve packages, or reproduce a language server's project state. Keep related modules, package files, tests, and documentation together when broader context matters.

History of .hs

Haskell began with a committee meeting in September 1987 that sought a common, open language for lazy functional programming. Haskell 1.0 followed in April 1990.⁵ The compact .hs suffix became the conventional name for ordinary Haskell source and remains the input suffix recognized by GHC.

The language later stabilized around Haskell 98 and Haskell 2010. The suffix does not declare which language edition or GHC extensions a file needs. Those details come from the source, compiler flags, and project configuration.

Working with .hs and related files

Haskell tooling uses several similar-looking suffixes:

GHC normally expects an importable module such as Data.Tree in a matching path such as Data/Tree.hs.⁶ Keep source and package configuration in version control, but treat .hi and .o files as reproducible build output. Use GHC, Cabal, or Stack to build the project instead of changing extensions.