I was running into a strange error that boils down to this
Myfunction[List_]:= Transpose[{List}]
Test = {1, 2};
Myfunction[Test]
The output is
Transpose[{1, 2}[{1, 2}]]
This doesn't make sense to me because if I just run
Transpose[{Test}]
I get
{{1}, {2}}
Can anyone explain why the test of {1, 2} appears twice when I run Myfunction?
This is because the FullForm of {…} is List[…]:
{aaa} // FullForm
(* List[aaa] *)
So, what you've defined is the same as
Myfunction[List_] := Transpose[List[List]]
To make it clearer, you've defined something equivalent to
Myfunction[a_] := Transpose[ a[a] ]
I'd say this is a good example showing why using built-in functions as variables is dangerous :) .
FullForm[Hold[Myfunction[List_] := Transpose[{List}]]]
Hold[SetDelayed[Myfunction[ Pattern[List, Blank[]]], (* 'List' is the name to substitute here *) Transpose[List[List]]]] (* Both 'List's are replaced in this expression *)
Or to be as specific as possible:
FullForm[{List}]
List[List]
You defined a substitution (Myfunction) which replaces all instances of List in its expression with the input. That it replaces both the explicit List inside the brackets and the implicit List provided by the brackets is expected (if not necessarily obvious or intuitive) behavior.
FullForm is useful for understanding things like this. It is also a good demonstration of why not to re-use built-in names (especially those with syntactic sugar that might hide them away) as parameter names.
List) as variable is bad practice, we don't have many questions about the possible side-effect caused by such bad practice, so +1. $\endgroup$Evaluation->Analyze notebook, you will get this issue clearly pointed out. i.sstatic.net/23noS0M6.png $\endgroup$