wolfram mathematica - How to make an analog of InString[]? -

i have discovered instring[] not work in mathlink mode when sending input enterexpressionpacket header. need define own function returns previous input line. 1 way have developed here not work in cases:

in[1]:= unevaluated[2 + 2] with[{line = $line - 1}, holdform[in[line]]] /. (downvalues[in]) out[1]= unevaluated[2 + 2] out[2]= 2 + 2 

this because ruledelayed has no holdallcomplete attribute. adding attribute makes ok:

in[1]:= unprotect[ruledelayed]; setattributes[ruledelayed, holdallcomplete]; protect[ruledelayed]; unevaluated[2 + 2] with[{line = $line - 1}, holdform[in[line]]] /. downvalues[in]  out[4]= unevaluated[2 + 2]  out[5]= unevaluated[2 + 2] 

but modifying built-in functions not idea. there better way this?

it seems have solved problem. here function:

in[1]:= getlastinput := module[{num, f},     f = function[{u, v},         {u /. {in -> num, holdpattern -> first}, holdform[v]}, holdallcomplete];     first@cases[         block[{ruledelayed = f}, downvalues[in]],         {$line - 1, x_} -> x, {1}, 1]]  in[2]:= unevaluated[2+2] getlastinput  out[2]= unevaluated[2+2]  out[3]= unevaluated[2+2] 

and have got answer question on instring in mathlink mode todd gayley (wolfram research):

instring assigned when using entertextpacket, not enterexpressionpacket. there no string form of input when sending enterexpressionpacket (whose content is, definition, expression).

edit:

i have found code not work input expressions head evaluate. solution replace holdform holdcomplete in code:

getlastinput := module[{num, f},     f = function[{u, v},         {u /. {in -> num, holdpattern -> first}, holdcomplete[v]}, holdallcomplete];     first@cases[         block[{ruledelayed = f}, downvalues[in]],         {$line - 1, x_} -> x, {1}, 1]] 

this works well. approach unprotect holdform , set attribute holdallcomplete on it. i'm wondering why holdform not have attribute default?

edit 2:

in comments main question leonid shifrin suggested better solution:

getlastinput :=   block[{ruledelayed},setattributes[ruledelayed,holdallcomplete];   with[{line=$line-1},holdcomplete[in[line]]/.downvalues[in]]] 

see comments details.

edit 3: last code can made better replacing holdcomplete double holdform:

getlastinput :=   block[{ruledelayed},setattributes[ruledelayed,holdallcomplete];   with[{line=$line-1},holdform@holdform[in[line]]/.downvalues[in]]] 

the idea taken presentation robby villegas of wolfram research @ 1999 developer conference. see subsection "holdcompleteform: non-printing variant of holdcomplete (just holdform hold)" in "working unevaluated expressions" notebook posted here.