Re: Java arrays to Scheme

[Per Bothner <per at bothner dot com>]

On 02/03/2017 12:33 PM, Peter Lane wrote:
> Hi list,
>
> I'm trying to understand how to interact with Java functions which return arrays, e.g. double[][] or int[].
>
> When I call a function which returns an int[] or double[][] they display as numbers within [ ... ] square brackets.  I can extract their length and elements, e.g. using (x 0) etc where x is the array.
>
> What kind of object are these in the Scheme universe?  array? vector?

They are Java arrays.  They are not Scheme vectors or arrays.
However, they are "vector-like", and can be converted to other sequence types
using splices.

(The '!' operator is roughly the same as define-constant.  Using either
improves type inference in the REPL.)

#|kawa:1|# (! ar (int[] 4 -5 6))
#|kawa:2|# ar
[4 -5 6]

> I would like to convert these Java arrays into a Scheme list or vector, which should be a simple function to write.  But is there a function/trick like that already?

#|kawa:3|# (vector @ar)
#(4 -5 6)
#|kawa:4|# (list @ar)
(4 -5 6)
#|kawa:5|# (s32vector @ar)
#s32(4 -5 6)
#|kawa:6|# (u32vector @ar)
#u32(4 4294967291 6)

Note the splice operator makes a copy - it does not share the array.

However, you can use the ->sequence cast to make them share.
(The syntax (->TYPE VALUE) is generally the same as (as TYPE VALUE).)

#|kawa:7|# (->sequence ar)
#s32(4 -5 6)
#|kawa:8|# (! seq (->sequence ar))
#|kawa:9|# seq
#s32(4 -5 6)
#|kawa:10|# (set! (ar 2) 16)
#|kawa:11|# ar
[4 -5 16]
#|kawa:12|# seq
#s32(4 -5 16)

#|kawa:13|# (set! (seq 0) 8)
java.lang.ClassCastException: gnu.math.IntNum cannot be cast to java.lang.Integer
	at gnu.lists.S32Vector.setRaw(S32Vector.java:10)
	at gnu.lists.AbstractSequence.set(AbstractSequence.java:214)
	at gnu.kawa.functions.Setter$SetList.apply2(Setter.java:133)
	at gnu.mapping.Procedure.apply(Procedure.java:138)
	at gnu.mapping.Procedure.apply(Procedure.java:118)
	at gnu.mapping.CallContext.runUntilDone(CallContext.java:227)
	at gnu.expr.ModuleExp.evalModule2(ModuleExp.java:350)
	at gnu.expr.ModuleExp.evalModule(ModuleExp.java:212)
	at kawa.Shell.run(Shell.java:283)
	at kawa.Shell.run(Shell.java:196)
	at kawa.Shell.run(Shell.java:183)
	at kawa.repl.processArgs(repl.java:714)
	at kawa.repl.main(repl.java:820)

Ooops - that really should work.  But we can use a cast:

#|kawa:14|# (set! (seq 0) (->int 8))
#|kawa:15|# seq
#s32(8 -5 16)
#|kawa:16|# ar
[8 -5 16]

Casting to a u32vector is trickier:

#|kawa:17|# (->u32vector ar)
/dev/stdin:17:14: warning - type int[] is incompatible with required type u32vector
java.lang.ClassCastException: [I cannot be cast to gnu.lists.U32Vector
	at atInteractiveLevel-17.run(stdin:17)
	at gnu.expr.ModuleExp.evalModule2(ModuleExp.java:293)
	at gnu.expr.ModuleExp.evalModule(ModuleExp.java:212)
	at kawa.Shell.run(Shell.java:283)
	at kawa.Shell.run(Shell.java:196)
	at kawa.Shell.run(Shell.java:183)
	at kawa.repl.processArgs(repl.java:714)
	at kawa.repl.main(repl.java:820)

Oops - that really should work, too ...

You can use a raw constructor, though:

#|kawa:19|# (gnu.lists.U32Vector ar)
#u32(8 4294967291 16)
--
	--Per Bothner
per@bothner.com   http://per.bothner.com/