Export["test.dat", {#, Sin[#]} & /@ Range[0, 6.1, .01]];

importFunction[fn_?FileExistsQ] := Module[{list},
   list = Import[fn, "Table"];
   Interpolation[list]
   ];

importFunction2[fn_?FileExistsQ] := Module[{list},
   list = Import[fn, "Table"];
   list[[#]] &
   ];

DeleteFile["test.dat"];
testfun[.17]
testfun2[18]
Sin[.17]
(* 0.169182 *)
(* {0.17, 0.169182} *)
(* 0.169182 *)

?testfun2

testfun2=list$5210[[#1]]&