Haskellismostlikelyquitedifferentfromanylanguageyouveeverusedbefore.Comparedtotheusualsetofconceptsinaprogrammersmentaltoolbox，functionalprogrammingoffersusaprofoundlydifferentwaytothinkaboutsoftware.InHaskell，wedeemphasizecodethatmodifiesdata.Instead，wefocusonfunctionsthattakeimmutablevaluesasinputandproducenewvaluesasoutput.Giventhesameinputs，thesefunctionsalwaysreturnthesameresults.Thisisacoreideabehindfunctionalprogramming.