> type 'a t = {l: float}
>
> Any thoughts ?
I think the crucial question is when new record types are born. Here is
my opinion:
The "=" sign in the above type mapping definition is what I would call
"delayed binding". "Early binding" would be equivalent to
type tmp = {lab : float}
type 'a s = tmp
(evaluate the right-hand side first, then define the mapping).
The "early binding" creates only one record type, so lab becomes
ordinary record label.
In the given example of the "delayed binding" the t becomes a machine
producing new record types.
Hence, the identifier l is not an ordinary record label. It is shared by
whole family of record types. We can see it this way:
# type 'a t = { la : float } ;;
type 'a t = { la : float; }
# {la = 0.};;
- : 'a t = {la = 0.}
So OCaml interpreter doesn't know the exact type of the last expression,
but it is clever enough to give it a generalized type.
We can use la to construct records of incompatible types:
# type 'a t = { la : float } ;;
type 'a t = { la : float; }
# let yy = ({la = 0.} : int t) ;;
val yy : int t = {la = 0.}
# let xx = ({la = 0.} : string t);;
val xx : string t = {la = 0.}
# xx = yy;;
Error: This expression has type int t but an expression was expected of type
string t
I suppose my jargon may be not mainstream, apologies.
Dawid
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