The readonly indexing function is used to create a table for which assigning new entries is illegal. Any object can be initially stored in a table that uses the readonly indexing function, but trying to into the table will raise an exception.

$\mathrm{table}\left(\'\mathrm{readonly}\'\,\left[\right]\right)$

$table\left(\mathrm{readonly}\,\left[\right]\right)$

$\mathrm{table}\left(\'\mathrm{readonly}\'\,\left[a\=1\,b\=2\right]\right)$

Error, (in `index/readonly`) cannot assign to a readonly table

$T≔\mathrm{subsop}\left(1\=\'\mathrm{readonly}\'\,\mathrm{table}\left(\left[a\=1\,b\=2\right]\right)\right)$

$T≔table\left(\mathrm{readonly}\,\left[a=1\,b=2\right]\right)$

$T_a$

$1$

$T_c≔1$

Error, (in `index/readonly`) cannot assign to a readonly table

$T_a≔1$

Error, (in `index/readonly`) cannot assign to a readonly table