Interpreting []::[], []::[]::[] in sml

Question

I would like to ask how []::[] and []::[]::[] are interpreted conceptually in SML/NJ.

My thought :

I thought that []::[] would generates error or generates a empty list but in fact it generates val it = [[]] : 'a list list.

After I know that []::[] generates [[]] : 'a list list, I thought that []::[]::[] would generate [[[]]] : 'a list list list but in fact it generates [[],[]] : 'a list list

Answer 1

:: is right-associative: In a::b::c::[] a,b,c must be elements of the same type and

a::b::c::[] = a::(b::(c::[]))
            = a::(b::[c])
            = a::[b,c]
            = [a,b,c]

This remains true even if a,b,c are []:

[]::[]::[]::[] = [[],[],[]]