My questions is, "Why would a language designer consider allowing comparison between different data types?". Also, does this make more sense in a functional language?
For example, in erlang one can perform the following comparisons:
%% Tuples are greater than numbers
999999 < {1,2}.
true
%% Lists are greater than tuples
{90000} < [1].
true
%% Atoms are greater than numbers
1 < false.
true
In python 2.x as well,
p = lambda x,y: x+y
p > (1)
True
p < (1)
False
p == (1)
False
Though looks like the python community decided this wasn't a good idea after all:
objects of different types always compare unequal, and are ordered consistently but arbitrarily. [...] This unusual definition of comparison was used to simplify the definition of operations like sorting and the in and not in operators. source
From the Python 3 release note:
The ordering comparison operators (<, <=, >=, >) raise a TypeError exception when the operands don’t have a meaningful natural ordering. Thus, expressions like 1 < '', 0 > None or len <= len are no longer valid, and e.g. None < None raises TypeError instead of returning False. A corollary is that sorting a heterogeneous list no longer makes sense – all the elements must be comparable to each other. source
This kind of explains why, but I was wondering if there are other reasons to allow this, particularly in functional languages.
In dynamic languages it makes a certain amount of sense, as it's nice to be able to sort heterogeneous lists and build heterogeneous trees. And I think I would say that it's not so much functional languages where it's dubious as it is strongly typed languages, for obvious reasons.