showPerson signature definition in section 5.7 doesn't work (using Purescript 0.12)

[gcsolaroli]

Following the book running the examples with Purescript 0.12 I had an issue with code snippets on sections 5.7 (Record Patterns and Row Polymorphism).

When declaring the function 'showPerson' following code listed in the book

 showPerson :: { first :: String, last :: String } -> String

I was not able to use to print out details of records with extra fields.

In order to get the desired behaviour, I had to change the declared signature, with the signature show a few lines below as output of :type showPerson:

 showPerson :: forall r. { firstName :: String, lastName :: String | r } -> String

I haven't tested this issue with prior version of the compiler.