我如何找出类型，数据构造函数或类型类的定义位置？

Question

I'm trying to figure out where the LayoutClass type or data constructor is defined for XMonad https://hackage.haskell.org/package/xmonad-0.12/docs/XMonad-Core.html#t:LayoutClass .

I've also tried using ghci with no success:

Prelude> import XMonad.Core
Prelude XMonad.Core> :t LayoutClass

<interactive>:1:1: error:
    Data constructor not in scope: LayoutClass
Prelude XMonad.Core>

Answer 1

LayoutClass is not a data type or a type constructor. It is a typeclass. You can verify it by clicking the Source button to the right of a row starting with LayoutClass in an Instances table in the documentation. For example, this is the first line of code for row starting with LayoutClass Layout Window

instance LayoutClass Layout Window where

This means that Layout Window is an instance of the LayoutClass typeclass.

Answer 2

在您链接的行的右边有一个“源”链接，在本例中为https://hackage.haskell.org/package/xmonad-0.12/docs/src/XMonad-Core.html#LayoutClass

Answer 3

Hoogle is often used for this purpose: http://hoogle.haskell.org/?hoogle=LayoutClass

But it does not seem to index all available packages on hackage.

The above typeclass is indexed by the latest (in Alpha for now) Hoogle: http://hoogle.haskell.org/?hoogle=LayoutClass

You can also search the stackage Hoogle: https://www.stackage.org/lts-8.11/hoogle?q=LayoutClass