'be to'

Sometimes I get to hear 'be to-infinitive' expression.

such as
I am to visit somewhere
he is to blame as well
or somthing like that.. (you could recall a lot more expressions than me)

Are those expressions used just to make it easy and convenient to speak?

If so, what's exact meaning of 'be to' expression? will? can? must? Does it depend on context?

If not, what's the purpose? what's the function of the expression?



hope you help me

to blame means guilty,
always you have to add to ,

He's to blame for this
Whos's to blame for that?,
etc,

I have never ever hear
I'm to visit,
but i've heard
i'm going to visit

I know 'be to blame' is kind of idiom rather than 'be to' expression.
I selected wrong example? doesn't it apply to that expression?
then, in which specific case do you use 'be to' ?
I don't understand.

Ok,
now i think that it means the same than Going to,m

I'm going to met with her.
I'm to met with her.

It's a expression that indicate progressive future,
but please tell where you got that from?

"If not, what's the purpose? what's the function of the expression?"

I read that "be to" was a for used in 18th and 19th centuries and it meant "must" or "have to". It appears in some Sherlock Holmes' stories, but currently "be to" is not used at all. For example, in "A Study in Scarlet"

"His eyes fairly glittered as he spoke, and he put his hand over his heart and bowed as if to some applauding crowd conjured up by his imagination.

“You are to be congratulated,” I remarked, considerably surprised at his enthusiasm."

Take care