Roundtrips

[jennybc]

When possible, if you have read_thingy(), it is good to have write_thingy() and for x to be identical to read_thingy(write_thingy(x)) and to test for that. Find positive (and negative?) examples.

[hadley]

Possibly more general principles of aligning inputs and outputs. Maybe also related to function inverses?

[hadley]

A symmetry argument is appealing because symmetry implies that you only have to remember half the API.

Postel's law is also relevant:

Be conservative in what you do, be liberal in what you accept from others

So write_thingy(read_thingy()) might produce a different file because it's got you onto the golden path. There will be some good examples from tidyr with unnest() and friends.

[jennybc]

So maybe it's more accurate to say that read_thingy(write_thingy(x)) should be identical to read_thingy(write_thingy(read_thingy(write_thingy(x)))), to account for x that is not canonical but still valid.