Hugs (interpreter)

Hugs (''Haskell User's Gofer System''), also Hugs 98, is a bytecode interpreter for the functional programming language Haskell. Hugs is the successor to Gofer, and was originally derived from Gofer version 2.30b. Hugs and Gofer were originally developed by Mark P. Jones, now a professor at Portland State University.

Hugs comes with a simple graphics library. As a complete Haskell implementation that is portable and simple to install, Hugs is sometimes recommended for new Haskell users.

Hugs deviates from the Haskell 98 specification in several minor ways. For example, Hugs does not support mutually recursive modules. A list of differences exists.

The Hugs prompt (a Haskell REPL) accepts expressions for evaluation, but not module, type or function definitions. Hugs can load Haskell modules at start-up.

Examples

Extensible records

An example of "Typed records with extensibility", a non standard feature unique to Hugs.

module Main where

import Hugs.Trex

type Coord = Double
type Point2D = Rec (x::Coord, y::Coord)
type Point3D = Rec (x::Coord, y::Coord, z::Coord)

point2D = (x=1, y=1) :: Point2D

-- emptyRec :: Rec EmptyRow -- predefined

-- (x=1 , (y=1)) -- rec. extension
-- (x=v , rec) -- record value decomposition, pattern fields must be non empty
-- (x::type , rec) -- record type decomposition

-- (rec\z) in the context means ''rec'' does not contain field ''z''

-- add a field z with the same type as field x
addZCoord :: (r\z, r\x) => t -> Rec ( x::t , r) -> Rec ( x::t, z::t , r)
addZCoord z ( x = x , other) = (x = x, z = z , other)

point3D = addZCoord 3 point2D -- :: Point3D

-- admit any record with ''showable'' fields x and y
printXY :: (Show t, r\x, r\y) => Rec (x::t, y::t , r) -> IO ()
printXY point = putStrLn xy
-- with SML style field accessors ('#' prefix)
where xy = show (#x point) ++", "++ show (#y point)

incrementX :: (Num t, r\x) => Rec (x::t , r) -> Rec (x::t , r)
incrementX (x=v , rest) = (x=v+1 , rest)

main = do
let point3D' = incrementX point3D
printXY point2D
printXY point3D'

Running with H98 compatibility turned off to activate language extensions:

runhugs -98 test.hs

References

External links

* {{official website, http://www.haskell.org/hugs

Free Haskell implementations
Software using the BSD license