programming - How to achieve Set+Part like behaviour in custom Set function?

So I've been toiling away on my DataFrame package. I've been trying to get Set to work with it, but it seems it either can't be done (ie via UpValues), or is ill advised. So I decided to bite the bullet and just roll my own custom DFSet function. But I can't figure out how the get the evaluation to work like it does when you Set the Part of something.

This is my DFSet function:

DFSet[Part[DataFrame[DFMetadata[metastore_], DFData[store_]], name_String], data_List] :=
    (
        If[
            Length@data =!= metastore["Length"], 
            Abort[]
        ];
        If[
            !MemberQ[metastore["Names"], name], 
            metastore["Names"] = Append[metastore["Names"], name]

        ];
        store[name] = Developer`ToPackedArray@data;
    )

The idea is that it would be called like this:

DFSet[frame[["a"]], {1, 2, 3}]

The problem is that if I run it as is the Part bit will try to evaluate and that won't do. But if I set the attribute HoldFirst on DFSet frame won't evaluate to its OwnValue.

A test case with code:

(****Code needed for a working test case but not part of the problem****)

(*Helper functions*)
StringVectorQ[x_] := MatchQ[x,{__String}]
TopMatrixQ[x_] := Length[Union@(Length/@x)]==1
(*Constructor*)
DataFrame[names_?StringVectorQ, data_?TopMatrixQ] := 
    Module[
        {store, metastore, length},
        If[Length@First@data =!= Length@names, Abort[]];
        store = Unique[DFstore];
        metastore = Unique[DFmetastore];

        metastore["Length"] = Length@First@Transpose@data;
        metastore["Names"] = names;
        Thread[
            Set[#1,#2]&[store/@names, Developer`ToPackedArray/@Transpose@data]
        ];
        DataFrame[DFMetadata[metastore], DFData[store]]
    ] 
(*Pretty Formatting*)
Format[DataFrame[DFMetadata[metastore_], _DFData]] := 
    "DataFrame"[Length[metastore["Names"]]]

(*UpValue for Part*)
DataFrame /: Part[DataFrame[DFMetadata[metastore_], DFData[store_]], name_String] := 
    (
        If[!MemberQ[metastore["Names"], name], Abort[]];
        store[name]
    )
(**************************)
(****The actual problem****)
(*DFSet function*)
DFSet[Part[DataFrame[DFMetadata[metastore_], DFData[store_]], name_String], data_List] :=

    (
        If[
            Length@data =!= metastore["Length"], 
            Abort[]
        ];
        If[
            !MemberQ[metastore["Names"], name], 
            metastore["Names"] = Append[metastore["Names"], name]
        ];
        store[name] = Developer`ToPackedArray@data;

    )
(*******************)
(****A test case****)
(*Arrange*)
frame = DataFrame[{"a","b"},{{1,2},{1,2},{1,2}}]
(*Act*)
DFSet[frame[["c"]],{3,3,3}]
DFSet[frame[["b"]],{4,4,4}]
(*Assert*)
frame[["c"]]=={3,3,3}

frame[["b"]]=={4,4,4}
(****Output from successful test should be:
Out=DataFrame[2]
Out=True
Out=True
*)

Answer

I'm not trying to read and understand all of your code but I understand the heart of the matter to be evaluating an arbitrary symbol frame in frame[["a"]] but not Part, within your DFSet function.

To that end you could do something like this:

ClearAll[DFSet, DFSetInner]


SetAttributes[{DFSet, DFSetInner}, HoldFirst]

DFSet[LHS_, RHS_] := Block[{Part}, DFSetInner[#, RHS] & @ LHS]

frame = DataFrame[{"a", "b"}, {{1, 2}, {1, 2}, {1, 2}}]

DFSet[frame[["a"]], {1, 2, 3}]

DFSetInner[DataFrame[{"a", "b"}, {{1, 2}, {1, 2}, {1, 2}}][["a"]], {1, 2, 3}]

You would then move the primary definition to DFSetInner.

---

Leonid raises a concern about the unintended consequences of Blocking Part. It seemed to me that unless DataFrame itself relied upon Part that this was reasonable, and I wanted to allow for things such as:

x := frame[["a"]];

DFSet[x, {1, 2, 3}]

which do not work with Leonid's present definition. If one prefers not to Block Part in this way another approach is:

DFSet[obj_[[part__]], RHS_] := DFSetInner[#[[part]], RHS] & @ obj

This no longer works with x above and requires a literal Part in the LHS. (HoldFirst must be maintained for both symbols as it was set above.)

Also, if one does not wish to introduce another Symbol DFSetInner you can attach the original definition to the same symbol with a different syntax as Leonid did. (I felt it best to keep them separate since I was not including the full definition in my answer.) You could also drop the "syntactic sugar" of Part in the inner definition of you so choose. Additionally if following this method we can leverage the order of parameters and the HoldFirst attribute to automatically evaluate obj in the normal course of evaluation:

SetAttributes[DFSet, HoldFirst]

DFSet[obj_[[parts__]], RHS_] := DFSet[{parts}, obj, RHS]

DFSet[{name_String}, DataFrame[DFMetadata[metastore_], DFData[store_]], data_List] := . . .

---

This still does not work with x := frame[["a"]]; DFSet[x, {1, 2, 3}]. If such behavior is desired one might extend the definition using my step function.

SetAttributes[DFSet, HoldFirst]

DFSet[obj_[[parts__]], RHS_] := DFSet[{parts}, obj, RHS]

DFSet[other_, RHS_] := step[other] /. _[x_] :> DFSet[x, RHS]

DFSet[{name_String}, DataFrame[DFMetadata[metastore_], DFData[store_]], data_List] := . . .

Now DFSet[x, {1, 2, 3}] works without the unintended consequences of Blocking Part.

frame = DataFrame[{"a", "b"}, {{1, 2}, {1, 2}, {1, 2}}];

x := frame[["a"]];

DFSet[x, {1, 2, 3}]