Custom typeclass instance

New to Daml
1

I have a data type Book that is an instance of custom typeclass Printable that has a function print. The print function returns Text that I want to print using debug statement. However, I am not able to print it directly with

   debug $ print myBook -- this gives error

But if I first assign the function’s return value to a variable and then use debug, it works.

   printOut: Text = print myBook 
   debug printOut -- this works

Can you please suggest what could be the issue here, and why do I need to assign it to a variable before printing?

Here is the full code and the error statement:

data Book = Book with 
    title: Text
    author: Text 

class Printable i o where 
    print: i -> o 

instance Printable Book Text where 
    print aBook = "Title: " <> aBook.title <> " by " <> aBook.author

testPrintable = script do 
    let 
        myBook = Book with 
            title = "Learn you a Daml"
            author = "Digital Asset" 

        printOut: Text = print myBook 

    --debug $ print myBook -- this gives error
    debug printOut -- this works
 
    return()

The error message:

    • Ambiguous type variable ‘b0’ arising from a use of ‘debug’
      prevents the constraint ‘(Show b0)’ from being solved.
      Probable fix: use a type annotation to specify what ‘b0’ should be.
      These potential instances exist:
        instance (Show a, Show b) => Show (Either a b)
          -- Defined in ‘GHC.Show’
        instance Show BigNumeric -- Defined in ‘GHC.Show’
        instance Show (Numeric n) -- Defined in ‘GHC.Show’
        ...plus 30 others
        ...plus six instances involving out-of-scope types
        (use -fprint-potential-instances to see them all)
    • In a stmt of a 'do' block: debug $ print myBook
      In the first argument of ‘script’, namely
        ‘do let myBook = ...
            let printOut : Text = print myBook
            debug $ print myBook
            debug printOut
            ....’
      In the expression:
        script
          do let myBook = ...
             let printOut : Text = print myBook
             debug $ print myBook
             debug printOut
             ....typecheck
1 Like
2

Essentially, you have the problem with multi-parameter typeclasses described here; where the ambiguous variable in that article is m, you have o being ambiguous.

debug does not sufficiently constrain o; consider its signature.

The standard solution is exactly the one in the followup article, a “functional dependency” or “fundep”. However, keep in mind that this is dependent [sorry] on your specific typeclass semantics, i.e. using a fundep here will make it so each choice of i can have one and only one Printable, not a bunch with different o choices. (That is, after all, the promise you are making when you declare the fundep “o is determined by i”.)

Fundeps are commonly used in the Daml standard library, e.g. HasExercise.

3

Thanks @Stephen !

So is it correct to say that the problem is with the debug function and not the way I have defined the Printable type-class? I am not sure how I can ‘constrain’ o in its definition.

Note: my conclusion above is based on just your response. I will read up the articles you linked later as they look like some heavy-lifting!

4

@Neelam_Dwivedi
Here’s how you can do it

class Printable i o | i -> o where 
    print: i -> o
5

Thanks @a_putkov :star_struck: