hkts | A simple encoding of higher-kinded types in TypeScript | Natural Language Processing library

I'm no expert in io-ts, but looking at this more closely, I'm thinking it isn't a straightforward thing to implement. The methods in a Schemable build schemables out of other schemables. The type() method takes a properties object whose keys are the keys of the desired object, and whose properties are schemables for each of the properties. The partial() method takes the same properties object, but builds a schemable where the properties are optional. You can see how these allow you to build the schemable; you have a properties object with all the keys you need to test for.

But given a schemable for type T, it isn't possible in general to convert it into one for Partial. The properties object that was passed into type() is not accessible in its result. It's been lost; thrown away. If all you have, say, is a type guard function that returns true if and only if it's given a value of type T, you can't use that function to make another one that produces true if and only if it's given a value of type Partial. You need more details of T from some source outside of the guard function, such as that properties object which is gone now.

So you can't do a straightforward composition of schemables to get this done.

The only way to do this would be to write your own Schemable that stores enough information about the Schema that calls it so that it can return a new schema where, say, the top-level call to S.type() is replaced with a call to S.partial(). Even that wouldn't be perfect, if you want to see your partial distribute across unions or down into intersections. Essentially it would have to perform surgery on the Schema, unwrap a top-level call to S.type() to get at the lost properties object, and rewrap it with S.partial().

It's easy enough to make one that turns all calls to S.type() into S.partial() (just replace your old schemable with a new one whose type property is a copy of the partial property), but that would end up making any nested objects partial as well as the top-level one.

Here's the horrible thing I did that kind of seems to work: