[Warning: beginner with FGL, the Functional Graph Library.] My first program with Data.Graph.Inductive works fine on ghc but raises a type error with hugs, which strikes me as odd. % hugs -98 smalltest.hs __ __ __ __ ____ ___ _________________________________________ || || || || || || ||__ Hugs 98: Based on the Haskell 98 standard ||___|| ||__|| ||__|| __|| Copyright (c) 1994-2005 ||---|| ___|| World Wide Web: http://haskell.org/hugs || || Report bugs to: hugs-bugs@haskell.org || || Version: 20050308 _________________________________________ Hugs mode: Restart with command line option +98 for Haskell 98 mode ERROR "smalltest.hs":7 - Type error in application *** Expression : mkGraph mynodes myedges *** Term : myedges *** Type : [(Integer,Integer,[Char])] *** Does not match : [LEdge [Char]] ghc works: % ghc --make smalltest.hs Chasing modules from: smalltest.hs Compiling Main ( smalltest.hs, smalltest.o ) Linking ... % ./a.out 0:"Toto"->[("Couic",1),("Crac",2)] 1:"Tata"->[] 2:"Shadok"->[] 3:"Machin"->[] Here is the code: import Data.Graph.Inductive mynodes = [(0, "Toto"), (1, "Tata"), (2, "Shadok"), (3, "Machin")] myedges = [(0, 1, "Couic"), (0, 2, "Crac")] graph :: Gr String String graph = mkGraph mynodes myedges main = do print graph