DSL.ts

// https://medium.com/@reidev275/creating-a-type-safe-dsl-for-filtering-in-typescript-53fe68a7942e
export type Filter<A> =
  | { kind: 'Equals'; field: keyof A; val: A[keyof A] }
  | { kind: 'Greater'; field: keyof A; val: A[keyof A] }
  | { kind: 'Less'; field: keyof A; val: A[keyof A] }
  | { kind: 'And'; a: Filter<A>; b: Filter<A> }
  | { kind: 'Or'; a: Filter<A>; b: Filter<A> }

export const equals = <A, K extends keyof A>(
  field: K,
  val: A[K]
): Filter<A> => ({
  kind: 'Equals',
  field,
  val,
})

export const greater = <A, K extends keyof A>(
  field: K,
  val: A[K]
): Filter<A> => ({
  kind: 'Greater',
  field,
  val,
})

export const less = <A, K extends keyof A>(field: K, val: A[K]): Filter<A> => ({
  kind: 'Less',
  field,
  val,
})

export const and = <A>(a: Filter<A>, b: Filter<A>): Filter<A> => ({
  kind: 'And',
  a,
  b,
})

export const or = <A>(a: Filter<A>, b: Filter<A>): Filter<A> => ({
  kind: 'Or',
  a,
  b,
})

export const interpretToSql = <A>(dsl: Filter<A>): string => {
  switch (dsl.kind) {
    case 'Equals':
      return `[${dsl.field}] = '${dsl.val}'`
    case 'Greater':
      return `[${dsl.field}] > '${dsl.val}'`
    case 'Less':
      return `[${dsl.field}] < '${dsl.val}'`
    case 'And':
      return `(${interpretToSql(dsl.a)} and ${interpretToSql(dsl.b)})`
    case 'Or':
      return `(${interpretToSql(dsl.a)} or ${interpretToSql(dsl.b)})`
  }
}

type JobPosting = {
  manager: string
  salary: number
}

const query: Filter<JobPosting> = and(
  equals('manager', 'Bob Slydell'),
  greater('salary', 50000)
)

console.log(interpretToSql(query))