Constant functions

[nthiery]

This trivial patch adds basic support for constant functions

Such a function could be written as lambda x: constant, but that's not picklable. Besides, this should eventually be cythoned for speed.

Prerequisite for #4326.

CC: @sagetrac-sage-combinat @mwhansen @jasongrout

Component: misc

Keywords: constant functions

Author: Nicolas M. Thiéry

Reviewer: Florent Hivert, Minh Van Nguyen

Merged: Sage 4.1.2.alpha0

Issue created by migration from https://trac.sagemath.org/ticket/6253

[nthiery]

comment:1

Oh, I forgot to mention: let me know if this readily exists somewhere and I missed it.

[mwhansen]

comment:2

How is this intended to be used? What are your typical constants? Are there instances where you'd want a non-constant function to be used in the same place?

[nthiery]

Description changed:

--- 
+++ 
@@ -2,3 +2,4 @@
 
 Such a function could be written as lambda x: constant, but that's not picklable. Besides, this should eventually be cythoned for speed.
 
+Prerequisite for #4326.

[nthiery]

comment:4

Replying to @mwhansen:

How is this intended to be used?

One of my use case looks like:

def my_objects(, predicate = ConstantFunction(True)):
"""
Returns all the objects blah blah blah (as an EnumeratedSet)
Optionally, a predicate can be specified to select only those objects satisfying the predicate

Another one looks like:

def generating_series(..., weight = ConstantFunction(1)):
...

What are your typical constants?

So far, True, 1, Integer(1)

Btw: with UniqueRepresentation, the two first yield the same constant function with the current implementation, thanks to this horror:

sage: { 1: 'a', True: 'b' }
{1: 'b'}

Fixed patch in a couple minutes.

Are there instances where you'd want a non-constant function to be used in the same place?

Yes. Actually, that's the case in all the situations I encountered so far

[nthiery]

Attachment: constant_function-6253-nt.patch.gz

Attachment: trac_6253-constant_function-nt.patch.gz

[nthiery]

Reviewer: Florent Hivert

[nthiery]

comment:5

The updated patch removes two unused imports spotted by Florent. Apply only this one.

[nthiery]

Changed author from nthiery to Nicolas M. Thiéry

[hivert]

comment:6

The patch looks good ! Positive review !

Florent