Hugs

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.[1] Hugs and Gofer were originally developed by Mark P. Jones, now a professor at Portland State University.

This article is about the Haskell interpreter. For the affectionate physical gesture, see Hug. For the confectionery, see Hershey's Hugs.
Hugs 98
Developer(s)Mark P. Jones, others
Stable release
September 2006 / September 21, 2006 (2006-09-21)
Operating systemCross-platform
TypeCompiler
LicenseBSD
WebsiteHugs 98 homepage

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[2] in several minor ways.[3] For example, Hugs does not support mutually recursive modules. A list of differences exists.[4]

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.[5]

Examples

Extensible records

An example of "Typed records with extensibility", a non standard feature unique to Hugs.[6]

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:[7]

runhugs -98 test.hs

References
  1. "Frequently Asked Questions about Hugs". Retrieved 2006-08-04.
  2. Simon Peyton Jones (editor) (December 2002). "Haskell 98 Language and Libraries: The Revised Report". Retrieved 2006-08-03.CS1 maint: extra text: authors list (link)
  3. "Haskell 98 non-compliance". The Hugs 98 User's Guide. Retrieved 2006-08-04.
  4. "List of differences with H98 standard".
  5. "Loading and editing Haskell module files". The Hugs 98 User's Guide. Retrieved 2006-08-04.
  6. "Hugs-specific language extensions". www.haskell.org.
  7. "Changing the behaviour of Hugs". www.haskell.org.
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