data structures - How to create strong types?

In a program I'm writing, I create a list of lists, that looks something like this:

{{100, {1,2,3,4,5}},
 {105, {2,4,6,8}},
 {42, {42,39,56}}}

I then pass this list of lists around to other functions. All well and good -- the problem is that this program has been developed iteratively, and this representation has changed, and so now I'd like to take more care in checking that the function is receiving the right kind of input.

In another language I could make this list of lists into its own type, and the compiler would enforce things. I know Mathematica has some facility for doing type checking, at least with primitive types, such as:

myFunc[a_String, b_Integer] := ...

But are there facilities available for creating (and enforcing, or at least checking) one's own ADTs?

Answer

In practice, enforcing strong types in Mathematica seldom pays off, just because, as mentioned by @belisarius, Mathematica is untyped (and perhaps more so than most other langauges, since it is really a term-rewriting system). So, most of the time, the suggestion of @Mr.Wizard describes what I'd also do.

The way to define ADT-s (strong types) was described in depth by Roman Maeder, in his books on Mathematica programming. This requires something more than what you provided in your question - namely, a more formal definition of what is in your data structure (so that we can form constructors, selectors and mutators). I will give here a very simple example to show how ADT can be implemented in Mathematica. The key points are using UpValues and (mostly inert) symbols to serve as heads of new types. Consider a simple "pair" type:

ClearAll[pair];

pair /: getFirst[pair[fst_, sec_]] := fst;
pair /: setFirst[pair[_, sec_], fst_] := pair[fst, sec];
pair /: getSecond[pair[fst_, sec_]] := sec;
pair /: setSecond[pair[fst_, _], sec_] := pair[fst, sec];

We can now define some function on this new type:

Clear[sortPairsByFirstElement];
sortPairsByFirstElement[pairs : {__pair}, f_] :=
     Sort[pairs, f[getFirst[#1], getFirst[#2]] &];

And here is an example of use:

pairs = Table[pair[RandomInteger[10],RandomInteger[10]],{10}]

{pair[0,10],pair[4,7],pair[5,3],pair[10,9],pair[9,2],pair[6,10],pair[3,7], pair[4,2],pair[0,4],pair[3,9]}

 sortPairsByFirstElement[pairs,Less]

{pair[0,4],pair[0,10],pair[3,9],pair[3,7],pair[4,2],pair[4,7],pair[5,3], pair[6,10],pair[9,2],pair[10,9]}

You can enforce stronger typing on what can go into a pair. One thing I've done is to enforce that in the "constructor":

pair[args__] /; ! MatchQ[{args}, {_Integer, _Integer}] :=
    Throw[$Failed, pair];

The technique just described produces truly strong types, in contrast to the pattern-based typing. Both are useful and complementary to each other. One reason why such strong typing as described above is rarely used in Mathematica is that all the rest of the infrastructure usual for the strongly-typed languages (compiler, type system, smart IDE-s, type-inference) is missing here (so you'd need to construct that yourself), plus often this will induce at least some overhead. For example, we may wish to represent an array of pairs as a 2-dimensional packed array for efficiency, but here the pair type will get in the way, and we'd have to write extra conversion functions (which will induce an overhead, not to mention the memory-efficiency). This is not to discourage this type of things, but just to note that over-using them, you may lose some advantages that Mathematica offers.