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allow specifying common base type for unions

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This commit is contained in:
Florian Dold 2019-12-14 19:09:01 +01:00
parent 47b2b84135
commit a0b5aba71c
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GPG Key ID: D2E4F00F29D02A4B
1 changed files with 34 additions and 14 deletions
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44
src/util/codec.ts
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@ -118,10 +118,10 @@ class ObjectCodecBuilder<T, TC> {
}
}
class UnionCodecBuilder<T, D extends keyof T, TC> {
class UnionCodecBuilder<T, D extends keyof T, B, TC> {
private alternatives = new Map<any, Alternative>();
constructor(private discriminator: D) {}
constructor(private discriminator: D, private baseCodec?: Codec<B>) {}
/**
* Define a property for the object.
@ -129,7 +129,7 @@ class UnionCodecBuilder<T, D extends keyof T, TC> {
alternative<V>(
tagValue: T[D],
codec: Codec<V>,
): UnionCodecBuilder<T, D, TC | V> {
): UnionCodecBuilder<T, D, B, TC | V> {
this.alternatives.set(tagValue, { codec, tagValue });
return this as any;
}
@ -140,21 +140,36 @@ class UnionCodecBuilder<T, D extends keyof T, TC> {
* @param objectDisplayName name of the object that this codec operates on,
* used in error messages.
*/
build<R extends TC>(objectDisplayName: string): Codec<R> {
build<R extends TC & B>(objectDisplayName: string): Codec<R> {
const alternatives = this.alternatives;
const discriminator = this.discriminator;
const baseCodec = this.baseCodec;
return {
decode(x: any, c?: Context): R {
const d = x[discriminator];
if (d === undefined) {
throw new DecodingError(`expected tag for ${objectDisplayName} at ${renderContext(c)}.${discriminator}`);
throw new DecodingError(
`expected tag for ${objectDisplayName} at ${renderContext(
c,
)}.${discriminator}`,
);
}
const alt = alternatives.get(d);
if (!alt) {
throw new DecodingError(`unknown tag for ${objectDisplayName} ${d} at ${renderContext(c)}.${discriminator}`);
throw new DecodingError(
`unknown tag for ${objectDisplayName} ${d} at ${renderContext(
c,
)}.${discriminator}`,
);
}
return alt.codec.decode(x);
const altDecoded = alt.codec.decode(x);
if (baseCodec) {
const baseDecoded = baseCodec.decode(x, c);
return { ...baseDecoded, ...altDecoded };
} else {
return altDecoded;
}
},
};
}
}
@ -180,10 +195,12 @@ export function stringConstCodec<V extends string>(s: V): Codec<V> {
if (x === s) {
return x;
}
throw new DecodingError(`expected string constant "${s}" at ${renderContext(c)}`);
}
}
throw new DecodingError(
`expected string constant "${s}" at ${renderContext(c)}`,
);
},
};
}
/**
* Return a codec for a value that must be a number.
@ -234,8 +251,11 @@ export function mapCodec<T>(innerCodec: Codec<T>): Codec<{ [x: string]: T }> {
}
export class UnionCodecPreBuilder<T> {
discriminateOn<D extends keyof T>(discriminator: D): UnionCodecBuilder<T, D, never> {
return new UnionCodecBuilder<T, D, never>(discriminator);
discriminateOn<D extends keyof T, B>(
discriminator: D,
baseCodec?: Codec<B>,
): UnionCodecBuilder<T, D, B, never> {
return new UnionCodecBuilder<T, D, B, never>(discriminator, baseCodec);
}
}