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Is there a way to see Typeclass definition in ghci for a specific type?
For example, Maybe is defined like this:
instance Functor Maybe where
fmap f (Just x) = Just (f x)
fmap f Nothing = Nothing
Can I see this in ghci ?
When, I use :info in ghci, I get this:
Prelude> :i Maybe
data Maybe a = Nothing | Just a -- Defined in `Data.Maybe'
instance Eq a => Eq (Maybe a) -- Defined in `Data.Maybe'
instance Monad Maybe -- Defined in `Data.Maybe'
instance Functor Maybe -- Defined in `Data.Maybe'
instance Ord a => Ord (Maybe a) -- Defined in `Data.Maybe'
instance Read a => Read (Maybe a) -- Defined in `GHC.Read'
instance Show a => Show (Maybe a) -- Defined in `GHC.Show'
In the above output, I want to see how it is defined in Data.Maybe as an instance for Functor. Anyway to see that in ghci ?
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A typeclass defines a set of methods that is shared across multiple types. For a type to belong to a typeclass, it needs to implement the methods of that typeclass. These implementations are ad-hoc: methods can have different implementations for different types.
A typeclass is a sort of interface that defines some behavior. If a type is a part of a typeclass, that means that it supports and implements the behavior the typeclass describes. A lot of people coming from OOP get confused by typeclasses because they think they are like classes in object oriented languages.
The second line, deriving (Eq, Show) , is called the deriving clause; it specifies that we want the compiler to automatically generate instances of the Eq and Show classes for our Pair type. The Haskell Report defines a handful of classes for which instances can be automatically generated.
Type classes define interfaces that in Haskell's terms are called dictionaries. For instance, from the Ord class definition the compiler will create a dictionary that stores all the class methods.
No, it's not possible – not just for instances but for anything. GHC only registers the compiled version of a package, so the source code generally won't be available to ghci.
Probably, most often you'll be using stuff from Hackage; in that case it's very simple to find the source code of such instances by hoogling the module, locating the class or data declaration, and clicking on source.
When you don't have internet access or whatever else reason you can't hoogle online, you first need to find out in what package the module is included. The easiest way to do that:
$ ghc-pkg find-module Data.Maybe
/usr/local/haskell/lib/ghc-7.6.2/package.conf.d
base-4.6.0.1
haskell2010-1.1.1.0
~/.ghc/x86_64-linux-7.6.2/package.conf.d
Then, as I said, GHC doesn't know where the source code to these packages is located – in fact it might not even be available on your system! But if you've installed the package (or one that depends on it) with cabal install, it will be there, by default in ~/.cabal/packages/hackage.haskell.org/PᴀᴄᴋᴀɢᴇNᴀᴍᴇ (as a compressed archive, but that's not a big hurdle). Within the package project folder, you can simply locate the module via the directory structure, which represents the module hierarchy.
Other packages, like your example of Data.Maybe (package haskell2010), may have come right with your installation of GHC, e.g. the Haskell platform. In that case, I believe the easiest thing is to search there for the Haddock documentation file. In my case,
$ find /usr/local/haskell -name 'Data-Maybe.html' | head -n1 | xargs firefox
That'll open up the equivalent to what hoogle links you to (but on your local HD), where you can also browse the source code in a user-friendly way.
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