Question

在下面的代码中，您可以看到一些令人生畏的重复输入：

implicit def intEqualInstance
  [ root,
    path <: TypePath[root] ]
  ( implicit mappingResolver: rel.rules.MappingResolver[path] )
  = {
    new genExp.Compiler
      [ genExp.templates.Where.Comparison[root, path, genExp.templates.Operator.Equal, typeLevel.Bool],
        genExp.values.Where.Comparison[ genExp.values.Expression.Value[ Int ] ],
        relExp.templates.Where,
        List[rel.Value] ]
      {
        override def compileTemplate(tpl: genExp.templates.Where.Comparison[root, path, genExp.templates.Operator.Equal, typeLevel.Bool]) = {
          ...
        }
        override def processValues(vals: genExp.values.Where.Comparison[ genExp.values.Expression.Value[ Int ] ]) = {
          ...
        }
      }
  }

我想为重复的类型声明类型别名。

我已尝试添加子类型约束，如下所示，但它使隐式解决方案失败：

implicit def intEqualInstance
  [ ...,
    inputTemplate <: genExp.templates.Where.Comparison[root, path, genExp.templates.Operator.Equal, typeLevel.Bool],
    inputValues <: genExp.values.Where.Comparison[ genExp.values.Expression.Value[ Int ] ] ]
  ( implicit mappingResolver: rel.rules.MappingResolver[path] )
  = {
    new genExp.Compiler
      [ inputTemplate, 
        inputValues, 
        ... ]
      {
        override def compileTemplate(tpl: inputTemplate) = ...
        override def processValues(vals: inputValues) = ...
      }
      ...

添加本地类型别名会使隐式解析失败：

implicit def intEqualInstance
  ...
  = {
    type InputTemplate = genExp.templates.Where.Comparison[root, path, genExp.templates.Operator.Equal, typeLevel.Bool]
    type InputValues = genExp.values.Where.Comparison[ genExp.values.Expression.Value[ Int ] ]
    new genExp.Compiler
      [ InputTemplate
        InputValues,
        ... ]
      ...

有没有解决方案？

Answer 1

我能想到的最好的方法是在外部范围内声明类型别名：

private type InputTemplate[root, path <: TypePath[root]] = 
  genExp.templates.Where.Comparison[root, path, genExp.templates.Operator.Equal, typeLevel.Bool]
private type InputValues = 
  genExp.values.Where.Comparison[ genExp.values.Expression.Value[ Int ] ]

implicit def intEqualInstance
  [ root, path <: TypePath[root] ]
  ( implicit mappingResolver: rel.rules.MappingResolver[path] )
  =
  new genExp.Compiler
    [ InputTemplate[root, path], InputValues, ... ]
    {
      override def compileTemplate(tpl: InputTemplate[root, path]) = ...
      override def processValues(vals: InputValues) = ...
    }

方法本地类型别名

1 个答案: