Cannot override property when extending structs

[rkmax]

Version

3.2.25

Expected behaviour

When extend a struct be able to overwrite props

Actual behaviour

Throws an error

Invalid call to mixin(target, source, [overwrite]): cannot overwrite property "X" of target object

Steps to reproduce

const t = require('tcomb');
const A = t.struct({a: t.String});
const B = t.struct({b: t.String});
const MyType = t.struct({
  myList: t.list(A)
});

const MyExtend =  MyType.extend({
  myList: t.list(B)
});

Stack trace and console log

/home/rkmax/my_project/node_modules/tcomb/lib/fail.js:2
  throw new TypeError('[tcomb] ' + message);
  ^

TypeError: [tcomb] Invalid call to mixin(target, source, [overwrite]): cannot overwrite property "worksheets" of target object
    at Function.fail (/home/rkmax/my_project/node_modules/tcomb/lib/fail.js:2:9)
    at assert (/home/rkmax/my_project/node_modules/tcomb/lib/assert.js:14:12)
    at mixin (/home/rkmax/my_project/node_modules/tcomb/lib/mixin.js:11:11)
    at /home/rkmax/my_project/node_modules/tcomb/lib/extend.js:50:5
    at Array.forEach (<anonymous>)
    at extend (/home/rkmax/my_project/node_modules/tcomb/lib/extend.js:43:10)
    at extendStruct (/home/rkmax/my_project/node_modules/tcomb/lib/struct.js:19:10)
    at Function.Struct.extend (/home/rkmax/my_project/node_modules/tcomb/lib/struct.js:112:12)
    at Object.<anonymous> (/home/rkmax/my_project/src/types/worksheet.js:189:46)
    at Module._compile (module.js:643:30)

[gcanti]

@rkmax actually this is intentional

[ArmorDarks]

Sounds like a special option like override which will allow to override them will be a good addition.

[rkmax]

then I'm misunderstanding the use of struct.extend functionality I was thinking in something like the extend of the classes where you can add or override things

[rodoabad]

@rkmax I wouldn't call that just purely "extending" anymore if you're overriding an existing property already.

[ArmorDarks]

@rodoabad why is that? In the wild extending does not imply that overriding isn't possible. Refer to the Classes, where overriding is a crucial part of the extending.