I have discovered that InString[]
does not work in MathLink
mode when sending input with EnterExpressionPacket
header. So I need to de
I would use $Pre and $Line
for this; unlike $PreRead
, it's applied to input expressions, not input strings or box forms. All you need is to assign it a function that has the HoldAllComplete
attribute, like this one which I've adapted from the example in the documentation:
SetAttributes[saveinputs, HoldAllComplete];
saveinputs[new_] :=
With[{line = $Line},
inputs[line] = HoldComplete[new]; new]
$Pre = saveinputs;
I tested this with MathLink, and the behavior seems to be what you desired (I've elided some of the transcript to highlight the key point):
In[14]:= LinkWrite[link,
Unevaluated[
EnterExpressionPacket[
SetAttributes[saveinputs, HoldAllComplete];
saveinputs[new_] :=
With[{line = $Line},
inputs[line] = HoldComplete[new]; new];
$Pre = saveinputs;]]]
In[15]:= LinkRead[link]
Out[15]= InputNamePacket["In[2]:= "]
In[20]:= LinkWrite[link,
Unevaluated[EnterExpressionPacket[Evaluate[1 + 1]]]]
In[21]:= LinkRead[link]
Out[21]= OutputNamePacket["Out[2]= "]
In[21]:= LinkRead[link]
Out[21]= ReturnExpressionPacket[2]
In[24]:= LinkWrite[link, Unevaluated[EnterExpressionPacket[DownValues[inputs]]]]
In[26]:= LinkRead[link]
Out[26]= ReturnExpressionPacket[
{HoldPattern[inputs[2]] :> HoldComplete[Evaluate[1 + 1]],
HoldPattern[inputs[3]] :> HoldComplete[DownValues[inputs]]}]
I just have found simpler but dangerous way:
In[3]:= Unevaluated[2 + 2]
Trace[In[$Line - 1]] // Last
Trace[In[$Line - 1]] // Last
Out[3]= Unevaluated[2 + 2]
Out[4]= Unevaluated[2 + 2]
During evaluation of In[3]:= $RecursionLimit::reclim: Recursion depth of 256 exceeded. >>
During evaluation of In[3]:= $RecursionLimit::reclim: Recursion depth of 256 exceeded. >>
During evaluation of In[3]:= $IterationLimit::itlim: Iteration limit of 4096 exceeded. >>
Out[5]= Hold[In[$Line-1]]
Does anybody know a way to make it safe?
It seems that I have solved the problem. Here is the 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]],
{$Line - 1, x_} -> x, {1}, 1]]
In[2]:=
Unevaluated[2+2]
getLastInput
Out[2]=
Unevaluated[2+2]
Out[3]=
Unevaluated[2+2]
And I just have got the answer to the question on InString
in MathLink
mode from Todd Gayley (Wolfram Research):
InString is only assigned when using EnterTextPacket, not EnterExpressionPacket. There is no string form of the input when sending EnterExpressionPacket (whose content is, by definition, already an expression).
EDIT:
I just have found that my code does not work with input expressions with head Evaluate
. The solution is to replace HoldForm
by HoldComplete
in my code:
getLastInput := Module[{num, f},
f = Function[{u, v},
{u /. {In -> num, HoldPattern -> First}, HoldComplete[v]}, HoldAllComplete];
First@Cases[
Block[{RuleDelayed = f}, DownValues[In]],
{$Line - 1, x_} -> x, {1}, 1]]
This works well. Another approach would be to unprotect HoldForm
and set up attribute HoldAllComplete
on it. I'm wondering why HoldForm
does not have this attribute by default?
EDIT 2:
In the comments for the main question Leonid Shifrin suggested much better solution:
getLastInput :=
Block[{RuleDelayed},SetAttributes[RuleDelayed,HoldAllComplete];
With[{line=$Line-1},HoldComplete[In[line]]/.DownValues[In]]]
See comments for details.
EDIT 3:
The last code can be made even better for by replacing HoldComplete
by double HoldForm
:
getLastInput :=
Block[{RuleDelayed},SetAttributes[RuleDelayed,HoldAllComplete];
With[{line=$Line-1},HoldForm@HoldForm[In[line]]/.DownValues[In]]]
The idea is taken from presentation by Robby Villegas of Wolfram Research at the 1999 Developer Conference. See subsection "HoldCompleteForm: a non-printing variant of HoldComplete (just as HoldForm is to Hold)" in "Working With Unevaluated Expressions" notebook posted here.