Haskell is a functional language similar to SML - the main difference is that Haskell is a lazy functional language. Here's a poem someone wrote for Haskell - most of it applies to SML as well!
Why should I renounce for you, dear Haskell,
My much yearned for side-effects?
Why should I face the software dragons
Without my weapon, my manly spear of destruction?
They call you non-strict, oh so elegant and pure Ariel.
Yet side-effect celibacy is surely severe.
Your flesh is too weak, you brutish beast.Alas, I can do without goto, without call/cc.
The tarpit demons of software hell await you!
This sinful habit in which you indulge
Does more harm than good.
Restrain yourself! And you too will see
The wondrous and refined joys of referential transparency!
Ah, flatter me not, you low-spirited Caliban!
Do you not know what advantages await
Those who renounce destructive update?
Start with an immaculate high-level specification,
Throw in some algebraic code transformation.
Soon you will have a provably correct and maintainable implementation.
Show mercy on mere mortals like me!
How I dream still of the efficient pleasures of pointer manipulation!
How I too wish to mutate memory with thoughts born of von Neumann earthiness!
Relent! Relent! Let me have my assignment, my printf, my gensym.
Let me fulfill my destructive impulses.
Let me set bang. Let me update. Let me assign. Let me mutate.
Fear not, lowly beast, I have heard your pleas.
To satisfy your low-level desire
I'll give you monads, linear types, MADTs,
Even single-threaded polymorphic lambda calculi.
My beauty may suffer, still I will aspire
To let you do (within typeful limits) what you please.
Rejoice! Rejoice! I'm free! I'm free!
The best of both worlds is mine at last.
Oh, infinite progeny of Church, Hope, and ML,
I curry favor not when I say:
Scan me right, fold me left,
Lazy lady of many shapes, you've got class.