Define nonstandard expansion rule for lists

Question

I have defined my own algebra using the unused operators $\oplus$ and $\otimes$. During evaluation, lists pop up which have the structure

{a__,{b_\[CirclePlus]c_},d__}

which I would like to expand to

{a__,b_,d__}\[CirclePlus]{a__,c_,d__}

I tried

{a_, {b_\[CirclePlus]c_}, d_} := {a, b, d}\[CirclePlus]{a, c, d}

but this returns the error

SetDelayed::shape: Lists {a__,{b_\[CirclePlus]c_},d__} and {a,b,d}\[CirclePlus]{a,c,d} are not the same shape.

How can I define such an expansion rule?

That's a specific problem with List. It will work with most other heads, e.g., list[a_, list[b_\[CirclePlus]c_], d_] := list[a, b, d]\[CirclePlus]list[a, c, d] is fine... — Henrik Schumacher, Commented Mar 10, 2018 at 10:45

Andrew · Accepted Answer · 2018-03-10 11:05:35Z

0

This works:

CirclePlus /: {a__, b_\[CirclePlus]c_, d__} := {a, b, d}\[CirclePlus]{a, c, d}

Without the internal list though.

answered Mar 10, 2018 at 11:05

Andrew

2,52317 silver badges15 bronze badges

Add a comment |

1 Answer 1