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I need to make the following data type an instance of Show:
data Tree a b = Branch b (Tree a b) (Tree a b) | Leaf a
I'm fairly new at this, but to start with, I interpreted this declaration as
"We have made a new type called Tree, which is parameterized with types a and b. A tree can be one of two things: a Branch which holds a piece of data of type b, plus two more Trees, or a Leaf which holds a data item of type a."
Now, I need to make a way to "Show" it nicely (nested branches, etc.), without using deriving. So far, I have only been writing functions in a module Main and loading/playing with them in an interpreter window, so I haven't actually done things with constructors, etc, before. Nevertheless, I figured I could just start off by declaring the tree data type within my file, as shown at the beginning of the question, and go from there.
As I messed around with "Show" without much success, I thought maybe I needed to define a small component of the tree and how to "Show" it first, before trying to work with the whole tree:
data Leaf a = Leaf a
instance Show (Leaf a) where
show (Leaf a) = ???
I tried a number of things in the ??? spot, such as "a", just a by itself, putStrLn, etc., but none are printing out the value of a when I say something like
>show (Leaf 3)
In fact, I have in many cases encountered this, which probably means I am not locating things right:
Ambiguous occurrence `show'
It could refer to either `Main.show', defined at a2.hs:125:1
or `Prelude.show',
imported from `Prelude' at a2.hs:2:8-11
(and originally defined in `GHC.Show')
...which I addressed by calling "Main.show," which of course doesn't work.
I guess the question is, where do I go with all this...or maybe just, "How can I fix the Leaf "Show" utility so that I can figure out how to extend it?" (assuming I have to define it first...)
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If you actually want to define an instance, then you, well, instantiate n with N : instance Printable N where print n = ... read str = ... At this point the type signatures are all fixed (from the class definition) and what you need to write are the actual bindings of these functions, hence it must be = , not :: .
An instance of a class is an individual object which belongs to that class. In Haskell, the class system is (roughly speaking) a way to group similar types. (This is the reason we call them "type classes"). An instance of a class is an individual type which belongs to that class.
A variable is said to be an instance of a data type. In C#, there are two main forms of data types, values and references. Unlike Java, C# does not have primitive data types, such as int. In C#, all data types are objects.
The shows functions return a function that prepends the output String to an existing String . This allows constant-time concatenation of results using function composition.
You have to start like that:
data Tree a b = Branch b (Tree a b) (Tree a b) | Leaf a
instance (Show a, Show b) => Show (Tree a b) where
show (Leaf x) = show x
show (Branch p l r) = ???
In order to show a Tree a b you have to first be able to show as and bs. That's what the (Show a, Show b) => part does, it specifies the preconditions you require for your instance to work.
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