How to parse to custom type in Haskell

Question

I'm new to haskell and have no idea how to parse.

I have created a new type

type Field = String

Now I want parse [String] to [Field], how could I do this? I already have this:

parse :: [String] -> [Field]

But here you do not parse: a Field is a String: you have defined a type alias. So that means that [String] is [Field]. You do not have to do any processing. — willeM_ Van Onsem
– willeM_ Van Onsem, Commented Sep 17, 2017 at 14:25

willeM_ Van Onsem · Accepted Answer · 2017-09-17 14:27:52Z

4

I have created a new type

No you have not. If you write type Foo = ... you define a type alias: there is no new type, you only have given a certain (composite) type a new name. From now on, you can use Field and String interleaved. You define new types with data Foo = ... or newtype Foo = .... Usually type Foo = ... is used to give a more complex type a specific name. Like for instance type NamedStringCollection = (String,[String]).

Since String and Field are the same type, so are [String] and [Field]. As a result you do not need to do any processing. If you have a [String], this is at the same time a [Field].

You can write:

parse :: [String] -> [Field]
parse = id  -- not necessary

But the id simply returns what you give it. So id does not do any work.

answered Sep 17, 2017 at 14:27

willeM_ Van Onsem

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