Sunday, June 4, 2023

Haskell 'newtype' and the record syntax

While reading some Haskell code snippets I found something that seemed confusing. The snippet involved newtype and record syntax. A simplified example is the following:

newtype PersonName = PersonName { theName :: String }
let p1 = PersonName $ getName obj
in print $ theName

I was not able to find a place where the PersonName was created by specifying the value of theName explicitly for example (Person { theName = "xyz" }) . The reason is that record syntax allows an alternative way of specifying the field values. For example:

let p1 = PersonName "Luis" -- valid!
let p2 = PersonName { theName = "Luis" }  -- valid!

Another thing that I found interesting is the newtype restrictions. For example it only allows you to specify one field in our record:

newtype PersonName' = PersonName' { firstName :: String, lastName ::String }

Compiling this is going to generate the following error:

newtypeexp.hs:6:23: error:
    • The constructor of a newtype must have exactly one field
        but ‘PersonName'’ has two
    • In the definition of data constructor ‘PersonName'’
      In the newtype declaration for ‘PersonName'’
6 | newtype PersonName' = PersonName' { firstName :: String, lastName ::String }
  |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

This seems to be related to the fact that newtype is used as a compile-type concept only. More info here .