[Haskell-beginners] type class question from Ternary Trees

[Chaddaï Fouché]

On Wed, Jul 29, 2009 at 4:28 PM, MH<mhamro at gmail.com> wrote:
> I am going over ternary tree library and it all looks clear to me with
> exception of the method below.
>
> <cut>
>
> What I don't understand here is recursive(?) call to 'get'. The first
> 'get' as I understand is an instance implementation of get from Binary
> module.

get is a method of the Binary typeclass, it is an action in the Get
monad. A "Get a" action can be run against a bytestring to transform a
binary code into an Haskell value of type a. A Binary instance
describe how to encode (with put) to binary records and decode (with
get) from binary records a specific type, here "TernarySet a".

Here you have the definition of get for a ternary tree, the get inside
this definition are call to get for the right type, not always the
same, for instance in :

> ch <- get

This get is of type "Get a" (of TernarySet _a_), this can be inferred
because ch is later used in "Node ch l e h". Similarly l, e and h are
determined to be of type "TernarySet a" and so the gets in :

> l <- if tag `testBit` 2 then get else return End
> e <- if tag `testBit` 1 then get else return End
> h <- if tag `testBit` 0 then get else return End

are of type "Get (TernarySet a)" and so they recursively call the get
we're defining (that is no surprise since this type is recursive).

getWord8 is one of the primitive provided by Binary, though you could
use get instead, it would often be unclear or need a type annotation
(like here).

If you have experienced the read function, this is the same idea,
though a little bit more advanced (one of these things you can't
easily do with object : be polymorphic on the return type).

Binary can achieve pretty good performances in good cases, sometimes
only limited by the speed of your hardware.

-- 
Jedaï