问题
The TemplateHaskell quoting documents two quotes ('') as the way to get the Name of a type:
> ''String
GHC.Base.String
This works fine for this type (name). However, I can't find a way to make it work nice for e.g. Maybe String:
> ''Maybe String -- interprets String as a data constructor
> ''Maybe ''String -- wants to apply ''String to the Name type
I know I can workaround via using [t| Maybe String |], but this is then in the Q monad, and requires type changes, and I think is not type-checked at the respective moment, only when spliced in.
I can also work around by first defining a type alias, type MaybeString = Maybe String, and then using ''MaybeString, but this is also cumbersome.
Any way to directly get what I want simply via the '' quotation?
回答1:
'' is used to quote names, not types. Maybe is a name, Maybe String is not. It is therefore not too surprising that you have to give your type a name by defining a type alias, before you can quote that name.
[t| |] on the other hand, quotes types. Note the difference here.
Prelude> :t ''String
''String :: Language.Haskell.TH.Syntax.Name
Prelude> :t [t| String |]
[t| String |]
:: Language.Haskell.TH.Syntax.Q Language.Haskell.TH.Syntax.Type
So I'm afraid you cannot use '' for what you're trying to do.
回答2:
I think what you're looking for is:
ConT ''Maybe `AppT` ConT ''String
来源：https://stackoverflow.com/questions/7489063/template-haskell-type-quoting-problems