On 09/29/2015 12:28 PM, Tim Roberts wrote:
I'm coming to Racket after many decades of programming in other
languages. One of the things that still gives me trouble is being able
to know exactly what type of "thing" I have at any given point.
Let me give you an example, which is actually quire similar to a
question asked last December. I've been playing with the database
module to read through a Mysql database. If I use query-rows, I get a
list of vectors. That's fine -- I know how to enumerate through the
list, and I know how to access the columns in the vector. But if I use
in-query, then I don't know what I'm getting. I get back a stream, but
a stream of what?
Here's an example.
(define rset (query-rows dbc "SELECT id,name,council FROM clubs
LIMIT 2"))
(length rset)
(first rset)
(define rset1 (in-query dbc "SELECT id,name,council FROM clubs LIMIT
2"))
(sequence-length rset1)
(sequence-ref rset1 0)
If I run that, the output is:
2
'#(35 "Salem Swingin' Stars" "Mid-Willamette Council")
2
35
"Salem Swingin' Stars"
"Mid-Willamette Council"
Clearly, the first thing is a vector. Easy-peasy. What's the second
thing? What type of thing prints as 3 separate entities with no
apparent container? It's true that I can iterate through the items by
doing:
(for ([(id name council) rset1]) (...))
but that only works if you know the number of columns beforehand. What
if I want to handle a variable number of columns? I foolishly assumed
that each entry in the sequence would be a vector, but this fails with
an arity mismatch:
(for ([row rset1]) (...))
as does this:
(define row (sequence-ref rset1 0))
Is in-query simply the wrong toy to use in that situation? What IS the
type of (sequence-ref rset1 0) in my example?
It's a multiple-valued sequence [1]; each iteration produces as many
values as the row has fields. Note that in Racket, "multiple values" is
not a first-class object.
You can receive multiple values using define-values (or let-values), if
you know how many to expect:
(define-values (id name council) (sequence-ref rset1 0))
Or you can deal with an arbitrary number of multiple values by using
call-with-values, eg to collect them in a vector:
(call-with-values
;; Thunk that produces multiple values:
(lambda () (sequence-ref rset1 0))
;; Variadic function that receives the values:
(lambda vs (list->vector vs))
(The latter function can be simplified to just `vector`, which takes an
arbitrary number of arguments and forms a vector containing them.)
Ryan
[1]
http://docs.racket-lang.org/guide/for.html#%28part._.Multiple-.Valued_.Sequences%29
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