A not-quite-functional way to simplify string interpolation conformances.

ExpressibleByAppendingStringInterpolation.swift

struct _AppendingStringInterpolation<Result: ExpressibleByStringInterpolation> {
  var result: Result
}
 
protocol ExpressibleByAppendingStringInterpolation: ExpressibleByStringInterpolation where StringInterpolation == _AppendingStringInterpolation<Self> {
  init(stringLiteral: String)
  mutating func append(_ value: Self)
}

extension ExpressibleByAppendingStringInterpolation where StringInterpolation == _AppendingStringInterpolation<Self> {
  init(stringInterpolation value: _AppendingStringInterpolation<Self>) {
    self = value.result
  }
}

extension _AppendingStringInterpolation: StringInterpolationProtocol where Result: ExpressibleByAppendingStringInterpolation {
  init(literalCapacity: Int, interpolationCount: Int) {
    var string = ""
    string.reserveCapacity(literalCapacity + interpolationCount * 2)
    result = Result(stringLiteral: string)
  }

  mutating func appendLiteral(_ literal: Result.StringLiteralType) {
    result.append(Result(stringLiteral: literal))
  }

  mutating func appendInterpolation(_ similar: Result) {
    result.append(similar)
  }
}


// *********************

typealias SQLValue = String       // Stub

struct QueryPart {
  var sql: String
  var values: [SQLValue]
}
 
extension QueryPart: ExpressibleByAppendingStringInterpolation {
  init(stringLiteral value: String) {
    self.init(sql: value, values: [])
  }
  
  mutating func append(_ part: QueryPart) {
    sql += part.sql
    values += part.values
  }
}

extension QueryPart.StringInterpolation {
  mutating func appendInterpolation(raw: String) {
    result.append(QueryPart(sql: raw, values: []))
  }
  
  mutating func appendInterpolation(param: SQLValue) {
    let placeholder = "$\(result.values.count + 1)"
    result.append(QueryPart(sql: placeholder, values: [param]))
  }
}