`InferOutput` behavior for nullish fields is inconsistent.

[cometkim]

const EnvSchema = v.object({
  STAGE: v.pipe(
    v.nullish(v.picklist(['alpha', 'prod'])),
    v.transform(input => {
      switch (input) {
        case 'alpha': return 'alpha';
        case 'prod': return 'production';
        default: return 'development';
      }
    }),
  ),
});

type EnvInput = v.InferInput<typeof EnvSchema>;
type EnvOutput = v.InferOutput<typeof EnvSchema>;

EnvSchema parses { STAGE?: "alpha" | "prod" | undefined | null } and transforms it to { STAGE: "alpha" | "production" | "development" }

So I expected the EnvOutput type to be { STAGE: "alpha" | "production" | "development" }, but it's { STAGE?: "alpha" | "production" | "development" | undefined }

This is the same for nullish and optional, but works as expected with nullable and undefinedable.

[fabian-hiller]

See #1095

[cometkim]

I can imagine it there might be a problem when it's optional, but I guess the different behavior between nullish and nullable & undefinable is a different issue?

[fabian-hiller]

Can you explain what you mean with a code example? I am happy to help and answer.

[cometkim]

Well, my actual use case is optional so I'm also facing the same issue described in #1095. But the behavior observed here is slightly different from it.

The EnvOutput with nullish field is inferred as

type EnvOutput = {
  STAGE?: "alpha" | "production" | "development" | undefined,
};

I expected nullish to mean T | null | undefined, but it acts like optional with transform.

So I changed the schema

const EnvSchema = v.object({
  STAGE: v.pipe(
    v.undefinedable(v.nullable(v.picklist(['alpha', 'prod']))),
    v.transform(input => {
      switch (input) {
        case 'alpha': return 'alpha';
        case 'prod': return 'production';
        default: return 'development';
      }
    }),
  ),
});

then correctly inferred as I expected:

type EnvOutput = {
  STAGE: "alpha" | "production" | "development",
};

[fabian-hiller]

How about you write this instead?

import * as v from 'valibot';

const envIndex = {
  alpha: 'alpha',
  prod: 'production',
  dev: 'development',
} as const;

const EnvSchema = v.object({
  STAGE: v.pipe(
    v.optional(v.picklist(['alpha', 'prod', 'dev']), 'dev'),
    v.transform((input) => envIndex[input])
  ),
});