New rule: io-ts codec restrictions

[danielnixon]

• Prevent use of t.type unless it is nested within a t.readonly.
• Prevent use of t.array, with fixer to t.readonlyArray.

[danielnixon]

You could probably fix t.type to t.readonly(t.type(...)) too.

[danielnixon]

Similarly, we likely want to enforce t.exact / t.strict to avoid issues related to unsafe decoding, smuggling extra props, etc.

[adamlthomas]

• Encourage providing a value for the optional name when using t.type
  This makes it somewhat easier to grok errors returned from deeply nested codecs.

[danielnixon]

If we widen the scope of this to fp-ts generally, another useful rule might be forbidding lib imports.

Good:

import { flow } from 'fp-ts/function'

Bad:

import { flow } from 'fp-ts/lib/function'

[danielnixon]

Oh look, that already exists https://github.com/buildo/eslint-plugin-fp-ts/blob/main/docs/rules/no-lib-imports.md