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Obtaining head and parts without evaluation

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3

$begingroup$

I have been trying to write a routine MyHead and MyPart
which would have the following properties.

MyHead[2+3] 

evaluates to Plus

MyHead[(2+4)[10,11]] 

evaluates to 2+4 (and not 6).

MyPart[f[2+3,10+11],1] 

evaluates to 2+3

MyPart[f[2+3,10+11],2] 

evaluates to 10+11

Ideally, this could be used to deeply
dive into an expression. For example, it could find the local variables
in the module

g[x_] := Module[y,z,w,t];

from

Defintions[g]

I can't get it. Any suggestions?

Mark

evaluation hold

share|improve this question

asked 8 hours ago

Mark S.Mark S.

14322 bronze badges

$endgroup$

add a comment | 

3

$begingroup$

I have been trying to write a routine MyHead and MyPart
which would have the following properties.

MyHead[2+3] 

evaluates to Plus

MyHead[(2+4)[10,11]] 

evaluates to 2+4 (and not 6).

MyPart[f[2+3,10+11],1] 

evaluates to 2+3

MyPart[f[2+3,10+11],2] 

evaluates to 10+11

Ideally, this could be used to deeply
dive into an expression. For example, it could find the local variables
in the module

g[x_] := Module[y,z,w,t];

from

Defintions[g]

I can't get it. Any suggestions?

Mark

evaluation hold

share|improve this question

asked 8 hours ago

Mark S.Mark S.

14322 bronze badges

$endgroup$

add a comment | 

$begingroup$

I have been trying to write a routine MyHead and MyPart
which would have the following properties.

MyHead[2+3] 

evaluates to Plus

MyHead[(2+4)[10,11]] 

evaluates to 2+4 (and not 6).

MyPart[f[2+3,10+11],1] 

evaluates to 2+3

MyPart[f[2+3,10+11],2] 

evaluates to 10+11

Ideally, this could be used to deeply
dive into an expression. For example, it could find the local variables
in the module

g[x_] := Module[y,z,w,t];

from

Defintions[g]

I can't get it. Any suggestions?

Mark

evaluation hold

share|improve this question

asked 8 hours ago

Mark S.Mark S.

14322 bronze badges

$endgroup$

MyHead[2+3] 

MyHead[(2+4)[10,11]] 

MyPart[f[2+3,10+11],1] 

MyPart[f[2+3,10+11],2] 

g[x_] := Module[y,z,w,t];

Defintions[g]

share|improve this question

asked 8 hours ago

Mark S.Mark S.

14322 bronze badges

share|improve this question

asked 8 hours ago

Mark S.Mark S.

14322 bronze badges

asked 8 hours ago

Mark S.Mark S.

14322 bronze badges

asked 8 hours ago

Mark S.Mark S.

14322 bronze badges

2 Answers
2

active

oldest

votes

3

$begingroup$

You can capture but you can't return 2+4 unless you hold it somehow. I will use Hold for that purpose:

MyHead // Attributes = HoldAll;
MyHead[expr_] := MyPart[expr, 0]

MyPart // Attributes = HoldAll;
MyPart[expr_, spec__] := Hold[expr][[1, spec]]

It fits all your examples except it returns results wrapped with Hold. You can use HoldForm to not show it but the point holds.

share|improve this answer

edited 8 hours ago

answered 8 hours ago

Kuba♦Kuba

110k1212 gold badges217217 silver badges555555 bronze badges

$endgroup$

add a comment | 

2

$begingroup$

I tend to prefer Defer over Hold because it doesn't show in the output and the results can be used as new input without having to release the hold manually.

Other than that, the following is copied straight from Kuba's solution:

MyHead // Attributes = HoldAll;
MyHead[expr_] := MyPart[expr, 0]
MyPart // Attributes = HoldAll;
MyPart[expr_, spec__] := Defer[expr][[1, spec]]

MyHead[2 + 3]
(* Plus *)

MyHead[(2 + 4)[10, 11]]
(* 2 + 4 *)

MyPart[f[2 + 3, 10 + 11], 1]
(* 2 + 3 *)

MyPart[f[2 + 3, 10 + 11], 2]
(* 10 + 11 *)

share|improve this answer

answered 8 hours ago

RomanRoman

14.1k11 gold badge1919 silver badges5151 bronze badges

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add a comment | 

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3

$begingroup$

You can capture but you can't return 2+4 unless you hold it somehow. I will use Hold for that purpose:

MyHead // Attributes = HoldAll;
MyHead[expr_] := MyPart[expr, 0]

MyPart // Attributes = HoldAll;
MyPart[expr_, spec__] := Hold[expr][[1, spec]]

It fits all your examples except it returns results wrapped with Hold. You can use HoldForm to not show it but the point holds.

share|improve this answer

edited 8 hours ago

answered 8 hours ago

Kuba♦Kuba

110k1212 gold badges217217 silver badges555555 bronze badges

$endgroup$

add a comment | 

3

$begingroup$

You can capture but you can't return 2+4 unless you hold it somehow. I will use Hold for that purpose:

MyHead // Attributes = HoldAll;
MyHead[expr_] := MyPart[expr, 0]

MyPart // Attributes = HoldAll;
MyPart[expr_, spec__] := Hold[expr][[1, spec]]

It fits all your examples except it returns results wrapped with Hold. You can use HoldForm to not show it but the point holds.

share|improve this answer

edited 8 hours ago

answered 8 hours ago

Kuba♦Kuba

110k1212 gold badges217217 silver badges555555 bronze badges

$endgroup$

add a comment | 

$begingroup$

You can capture but you can't return 2+4 unless you hold it somehow. I will use Hold for that purpose:

MyHead // Attributes = HoldAll;
MyHead[expr_] := MyPart[expr, 0]

MyPart // Attributes = HoldAll;
MyPart[expr_, spec__] := Hold[expr][[1, spec]]

It fits all your examples except it returns results wrapped with Hold. You can use HoldForm to not show it but the point holds.

share|improve this answer

edited 8 hours ago

answered 8 hours ago

Kuba♦Kuba

110k1212 gold badges217217 silver badges555555 bronze badges

$endgroup$

MyHead // Attributes = HoldAll;
MyHead[expr_] := MyPart[expr, 0]

MyPart // Attributes = HoldAll;
MyPart[expr_, spec__] := Hold[expr][[1, spec]]

share|improve this answer

edited 8 hours ago

answered 8 hours ago

Kuba♦Kuba

110k1212 gold badges217217 silver badges555555 bronze badges

answered 8 hours ago

Kuba♦Kuba

110k1212 gold badges217217 silver badges555555 bronze badges

answered 8 hours ago

Kuba♦Kuba

110k1212 gold badges217217 silver badges555555 bronze badges

2

$begingroup$

I tend to prefer Defer over Hold because it doesn't show in the output and the results can be used as new input without having to release the hold manually.

Other than that, the following is copied straight from Kuba's solution:

MyHead // Attributes = HoldAll;
MyHead[expr_] := MyPart[expr, 0]
MyPart // Attributes = HoldAll;
MyPart[expr_, spec__] := Defer[expr][[1, spec]]

MyHead[2 + 3]
(* Plus *)

MyHead[(2 + 4)[10, 11]]
(* 2 + 4 *)

MyPart[f[2 + 3, 10 + 11], 1]
(* 2 + 3 *)

MyPart[f[2 + 3, 10 + 11], 2]
(* 10 + 11 *)

share|improve this answer

answered 8 hours ago

RomanRoman

14.1k11 gold badge1919 silver badges5151 bronze badges

$endgroup$

add a comment | 

2

$begingroup$

I tend to prefer Defer over Hold because it doesn't show in the output and the results can be used as new input without having to release the hold manually.

Other than that, the following is copied straight from Kuba's solution:

MyHead // Attributes = HoldAll;
MyHead[expr_] := MyPart[expr, 0]
MyPart // Attributes = HoldAll;
MyPart[expr_, spec__] := Defer[expr][[1, spec]]

MyHead[2 + 3]
(* Plus *)

MyHead[(2 + 4)[10, 11]]
(* 2 + 4 *)

MyPart[f[2 + 3, 10 + 11], 1]
(* 2 + 3 *)

MyPart[f[2 + 3, 10 + 11], 2]
(* 10 + 11 *)

share|improve this answer

answered 8 hours ago

RomanRoman

14.1k11 gold badge1919 silver badges5151 bronze badges

$endgroup$

add a comment | 

$begingroup$

I tend to prefer Defer over Hold because it doesn't show in the output and the results can be used as new input without having to release the hold manually.

Other than that, the following is copied straight from Kuba's solution:

MyHead // Attributes = HoldAll;
MyHead[expr_] := MyPart[expr, 0]
MyPart // Attributes = HoldAll;
MyPart[expr_, spec__] := Defer[expr][[1, spec]]

MyHead[2 + 3]
(* Plus *)

MyHead[(2 + 4)[10, 11]]
(* 2 + 4 *)

MyPart[f[2 + 3, 10 + 11], 1]
(* 2 + 3 *)

MyPart[f[2 + 3, 10 + 11], 2]
(* 10 + 11 *)

share|improve this answer

answered 8 hours ago

RomanRoman

14.1k11 gold badge1919 silver badges5151 bronze badges

$endgroup$

MyHead // Attributes = HoldAll;
MyHead[expr_] := MyPart[expr, 0]
MyPart // Attributes = HoldAll;
MyPart[expr_, spec__] := Defer[expr][[1, spec]]

MyHead[2 + 3]
(* Plus *)

MyHead[(2 + 4)[10, 11]]
(* 2 + 4 *)

MyPart[f[2 + 3, 10 + 11], 1]
(* 2 + 3 *)

MyPart[f[2 + 3, 10 + 11], 2]
(* 10 + 11 *)

share|improve this answer

answered 8 hours ago

RomanRoman

14.1k11 gold badge1919 silver badges5151 bronze badges

answered 8 hours ago

RomanRoman

14.1k11 gold badge1919 silver badges5151 bronze badges

answered 8 hours ago

RomanRoman

14.1k11 gold badge1919 silver badges5151 bronze badges