Created by [CecilWesterhof].

Default you cannot check if something is a dict, That is why I thought it a good idea to write a proc for it.

A simple check is to check if something is a list and if so if the list contains an even number of elements.

But I find this not good enough, because this would validate:
======
{[} \] {[} \]
======

In my opinion this is not a correct dict. (Opinions differ about this.) It can certainly not be created with dict operations. And when doing:
======
dict set d \{ }
======

The list is cleaned up and you get:
======
{[} \] \{ \}
======

That is why I check if the length of the list is two times the number of the keys.
The code becomes:
======
proc isDict {d} {
    expr {[string is list $d]      && \
              ! ([llength $d] % 2) && \
              ((2 * [llength [dict keys $d]]) == [llength $d])}
}
======

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As always: comments, tips and questions are appreciated.

[PL] 2018-07-22: see also TIP [https://core.tcl.tk/tips/doc/trunk/tip/501.md%|%501] "string is dict".

[EMJ] 2018-07-22: which I think would also validate the above four-element list.

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[EMJ] 2018-07-22: Put the original four-element list above in a variable and you can use that variable as a dict. Of course the duplicate key disappears because
that is a property of dicts (and if the values were different it is the last which would be taken).

Also the proc above will error if fed, for example, a three-element list, so you need a catch or try in there.

[CecilWesterhof] 2018-07-23: No, it will go OK:
======
$ set d [list a b c]
a b c
$ isDict $d
0
======

[EMJ] 2018-07-23: Ah, yes - lazy evaluation of && .

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[PYK] 2018-07-23:  This topic is also discussed at [dict tips and tricks].

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