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- [Codec interface](#codec-interface)

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# Codec interface

```ts
export interface Codec<I, O, A> extends D.Decoder<I, A>, E.Encoder<O, A> {}
```

A codec is just a decoder and an encoder packed together.

The following laws must hold

1. `pipe(codec.decode(u), E.fold(() => u, codec.encode)) = u` for all `u` in `unknown`
2. `codec.decode(codec.encode(a)) = E.right(a)` for all `a` in `A`

You can build a new codec using the `make` helper

**Example**

```ts
import * as C from 'io-ts/Codec'
import * as D from 'io-ts/Decoder'
import * as E from 'io-ts/Encoder'
import { pipe } from 'fp-ts/function'

const decoder: D.Decoder<unknown, number> = pipe(
  D.string,
  D.parse((s) => {
    const n = parseFloat(s)
    return isNaN(n)
      ? D.failure(s, `cannot decode ${JSON.stringify(s)}, should be parsable into a number`)
      : D.success(n)
  })
)

const encoder: E.Encoder<string, unknown> = {
  encode: String
}

export const NumberFromString: C.Codec<unknown, string, number> = C.make(decoder, encoder)
```