I was looking at this Q&A about using pattern test (pattern_?test) vs pattern condition (pattern_/;cond) and came across this example where using condition was the only possible way to restrict the function parameters (function definition added by me):

Clear[fCond]
fCond[x__ /; Plus[x] == 7] := {x}^2
fCond[1, 2, 4]
(* {1, 4, 16} *)

I know that, aside from the built-in tests in MMA (IntegerQ, NumericQ, eg), one can write custom pattern tests using pure functions [2]:

Hence, I was trying to be clever by using tests with sequence pattern (__) coupled with sequence argument for pure function (##). However, this did not work when plugging in values for the parameter/pattern.

Clear[fTest]
fTest[x__?(Plus[##] == 7 &)] := {x}^2
fTest[1, 2, 4]
(* fTest[1, 2, 4] *)

, even though the pure function test alone does work with the plugged in sequence

Plus[##] == 7 &[1, 2, 4]
(* True *)

Why doesn't pattern test work via this method? If so, is there any other way to do pattern test as opposed to pattern condition in this case?

[2]: Ruskeepaa's Mathematica Navigator 2009 p.498

I was looking at this Q&A about using pattern test (pattern_?test) vs pattern condition (pattern_/;cond) and came across this example where using condition was the only possible way to restrict the function parameters (function definition added by me):

Clear[fCond]
fCond[x__ /; Plus[x] == 7] := {x}^2
fCond[1, 2, 4]
(* {1, 4, 16} *)

I know that, aside from the built-in tests in MMA (IntegerQ, NumericQ, eg), one can write custom pattern tests using pure functions [2]:

Hence, I was trying to be clever by using tests with sequence pattern (__) coupled with sequence argument for pure function (##). However, this did not work when plugging in values for the parameter/pattern.

Clear[fTest]
fTest[x__?(Plus[##] == 7 &)] := {x}^2
fTest[1, 2, 4]
(* fTest[1, 2, 4] *)

, even though the pure function test alone does work with the plugged in sequence

Plus[##] == 7 &[1, 2, 4]
(* True *)

Why doesn't pattern test work via this method? If so, is there any other way to do pattern test as opposed to pattern condition in this case?

[2]: Ruskeepaa's Mathematica Navigator 2009 p.498

I was looking at this Q&A about using pattern test (p_?test) vs pattern condition (p_/;cond) and came across this example where using condition was the only possible way to restrict the function parameters (function definition added by me):

f3[x__] /; Plus[x] == 7 := {x}^2

I know that, aside from the built-in tests in MMA (IntegerQ, NumericQ, eg), one can write custom pattern tests using pure functions [2]:

Hence, I was trying to be clever by using tests with sequence pattern (__) coupled with sequence argument for pure function (##). However, this did not work when plugging in values for the parameter/pattern.

Clear[fTest]
fTest[x__?(Plus[##] == 7 &)] := {x}^2
fTest[1, 2, 4]

, even though the pure function test alone does work with the plugged in sequence

Plus[##] == 7 &[1, 2, 4]

Why doesn't pattern test work via this method? If so, is there any other way to do pattern test as opposed to pattern condition in this case?

PS: pattern condition works as expected

Clear[fCond]
fCond[x__ /; Plus[x] == 7] := {x}^2
fCond[1, 2, 4]

[2]: Ruskeepaa's Mathematica Navigator 2009 p.498

I was looking at this Q&A about using pattern test (pattern_?test) vs pattern condition (pattern_/;cond) and came across this example where using condition was the only possible way to restrict the function parameters (function definition added by me):

Clear[fCond]
fCond[x__ /; Plus[x] == 7] := {x}^2
fCond[1, 2, 4]
(* {1, 4, 16} *)

I know that, aside from the built-in tests in MMA (IntegerQ, NumericQ, eg), one can write custom pattern tests using pure functions [2]:

Hence, I was trying to be clever by using tests with sequence pattern (__) coupled with sequence argument for pure function (##). However, this did not work when plugging in values for the parameter/pattern.

Clear[fTest]
fTest[x__?(Plus[##] == 7 &)] := {x}^2
fTest[1, 2, 4]
(* fTest[1, 2, 4] *)

, even though the pure function test alone does work with the plugged in sequence

Plus[##] == 7 &[1, 2, 4]
(* True *)

Why doesn't pattern test work via this method? If so, is there any other way to do pattern test as opposed to pattern condition in this case?

[2]: Ruskeepaa's Mathematica Navigator 2009 p.498