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string monoid class should define one()

asked 2016-02-03 20:56:00 -0600

mark gravatar image

updated 2016-02-04 04:52:05 -0600

tmonteil gravatar image

Method 'one' is supposed to return the identity element of a monoid (or I guess of any magma). String monoids should define it to be the empty string. Not only have they not, the default implementation throws an error:

sage: BinaryStrings().one()

TypeError: Argument x (= 1) is of the wrong type.

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answered 2016-02-04 04:50:59 -0600

tmonteil gravatar image

updated 2016-02-04 04:51:48 -0600

Good catch ! Looking at the code, it seems that this class was not touched for a while (created in 2007, last touched in 2009), so i guess nobody took care of it when the one() was created for all monoids (probably through the sategory mechanism). May i encourage you to open a ticket and fix it yourself ?

Actually, it is not correct to say that the monoid is not able to provide an identity element, you can get is as follows:

sage: b('')

sage: b('110') == b('') * b('110')
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Since the OP may not have an account, I've taken the liberty of opening for this.

kcrisman gravatar imagekcrisman ( 2016-02-04 07:46:27 -0600 )edit

And indeed there is already a branch on it! Though it needs docstring/tests.

kcrisman gravatar imagekcrisman ( 2016-02-04 10:53:48 -0600 )edit

It is now positively reviewed, so the fix should enter the next Sage release.

tmonteil gravatar imagetmonteil ( 2016-02-12 16:24:16 -0600 )edit

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Asked: 2016-02-03 20:56:00 -0600

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Last updated: Feb 04 '16