On 11/03/2017 08:22 AM, Anthony Clayden wrote:
On 4/11/2017, at 12:57 AM, Adam Chlipala <[email protected]> wrote:
On 11/02/2017 09:51 PM, Anthony Clayden wrote:
I'm curious what the full signature would look like. Here's my guess:
fun natJoin [ t1' :: {Type} ] [ t1_2 :: {Type} ] [ t2' :: {Type} ]
[ t1' ~ t1_2 ] [ t1' ~ t2' ] [ t1_2 ~ t2' ]
( t1 : $( t1' ++ t1_2 ) ) ( t2 : $( t1_2 ++ t2' ) )
= ??
One possibility is:
t1 ++ (t2 --- t1_2)
IIUC that's merely projecting away the t1_2 attributes from t2; then
cross-multiplying with t1(?) What it needs is matching records type t1 with
records type t2 that have the same values in the fields in common -- i.e. in
their t1_2 fields. That's typically implemented as a record-by-record fold over
one argument.
It's not clear to me what result type you expect, though.
Oh, that was in the earlier message: [ t1' ++ t1_2 ++ t2' ]
Oh, you meant the brackets to signify a list, Haskell-style? But then
the argument types don't make sense, as they only give single records,
not lists or sets of records.
In real Ur/Web code, such a function would be a bad smell, since SQL
queries do all this natively.
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