CODEWITHSUNDEEP
haskell
Manoj R
Manoj R

Reputation: 3247

Empty List of specific type in Haskell

I need to create an empty list of specific type.
I am creating a stack which in internally a list. To create empty stack I need to pass empty list. But how can I pass a empty list of specific type. 

data Stack' a = Stack' [a] deriving (Show)
main = print("MyName", " ", Stack' Int [])

Instead of Int [], I have tried []::Int and various other combination with brackets. But no success. Currently I have settled with 

ls :: [Int]
ls = []
Stack' ls

Upvotes: 3

Views: 1924

Answers (1)

willeM_ Van Onsem
willeM_ Van Onsem

Reputation: 476624

Specifying an explicit type

You can set the type after double colons (::), like:

main = print ("MyName", " ", Stack' [] :: Stack' Int)

here we thus set that the type of the third parameter of the 3-tuple is an object with type Stack' Int.

or here you can also set the type at the list level (but these are equivalent):

main = print ("MyName", " ", Stack' ([] :: [Int]))

or more canonical:

main = print ("MyName", " ", Stack' ([] :: [] Int))

Here we thus specify "An empty list of type [Int]".

Note that this will print a the value with the parenthesis and commas of the 3-tuple:

Prelude> print ("MyName", " ", Stack' [] :: Stack' Int)
("MyName"," ",Stack' [])

If you want to print this without the "3-tuple noise", you should construct a string, for example by using show :: Show a => a -> String:

main = putStrLn ("MyName " ++ show (Stack' [] :: Stack' Int))

This then yields:

Prelude> putStrLn ("MyName " ++ show (Stack' [] :: Stack' Int))
MyName Stack' []

Type applications

We can also make use of a GHC extension: TypeApplication.

In that case we need to compile with the -XTypeApplication flag, or write {-# LANGUAGE TypeApplication #-} in the head of the file.

We can then specify the type of the type parameter a with the @Type notation, like:

main = print ("MyName", " ", Stack' @Int [])

Upvotes: 6

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