type inferrer

[justin seliger]

me:  Hey Jay, Robby and I had a question.  In the function alpha-vary, it says that hash-set and such will be useful, but we are wondering why don't we just type case on the expression and rewrite any id we find with a gensym?

instead of storing it that is

 Jay:  you have to make sure you change bound occurrences to the same thing you changed the binding occurrence to

 me:  OOooOOoo

 Jay:  ie, (fun (x : number) x) should be (fun (x111 : number) x111) not (fun (x111 : number) x234)

 me:  that makes sense

 Jay:  send this conversation to the mailing list, please

 me:  okay

thx

 Sent at 10:19 PM on Wednesday

 me:  Jay, So, we are making a hash... and using hash set.. but its not setting anything. and we are confused. and very sad.

We define the hash using make-immutable hasheq initializing it wiht empty

then do hash-set x (gensym x)

then hash ref x.... and we get nothing

we are also printing out the hash inbetween setting and reffing.. and it is empty

any idea why this is happening?

 Jay:  show me your program

 me:  i just emailed it to u

you*

 Jay:  hash-set returns a new hash table with the change

 me:  we also tried hash-set!~

 Jay:  you are looking at the old one from before the change

 me:  err hash-set!

 Jay:  you should use hash-set

just use it correctly

 me:  so we should re define the hash after each time we use hash-set

?

 Jay:  you could think of it that way

you should never change the hash except when new ids are introduced

never when they are used

 me:  I'm sorry, but i'm not sure i understand.

you mean that we should be using hash-set in the typecase of id

and just using alpha-vary everywhere else?

 Jay:  no

id is when ids are used

 Sent at 10:55 PM on Wednesday

 me:  ...

i dont understand

i'm sorry. but that jsut confused me more

just*

Could you explain that in different words again please?

 Jay:  "id" appears in programs when identifiers are used

eg (fun (x:number) x) the second x is an "id" data structure

because x is "used"

in contrast, the first x is when the identifier is introduced

 me:  ahh

okay.

what about the following:

(alpha-vary (parse 'x))

that should return

(id x)

well the parse should

thus the alpha vary should return

(id x12342)

so would that count as being introduced? or used?

 Jay:  that is being used

and never introduced

so it is an error

 me:  i see.

a parse error?

 Jay:  no an alpha-vary error