Print argument of arbitrary type

[PhucVH888]

I got this error message when I defined a method to print an argument of arbitrary type.

encorec -c test.enc
Importing module String from /Users/vo/Dropbox/code/encore/bundles/standard/St$
encorec: Expr.hs: typeToPrintfFstr not defined for t

Here is the example code. At first, I intended to post the question, however, I found the simple solution later, so I am not sure if it is suitable to post it here.

class Foo<t>{
  def printF(x:t) : void
    print x
}

class Main{
  def main() : void {
    let foo = new Foo<int>
    in foo.printF(10)
  }
}

Solution: Instead of using print x directly, I slightly change to print (Just x) and then it works well. Just in case others can have the same problem to me in the future.

class Foo<t>{
  def printF(x:t) : void
    print (Just x)
}

[TobiasWrigstad]

Your solution does not seem to be a good solution. Why should print x not work whereas print (Just x) works? I suggest fixing the root cause of this.

[PhucVH888]

Yes, you're right, I have checked that and the printing results of print x and print (Just x) are completely different from expectation.

[EliasC]

History repeats itself :) #46

I think printing a polymorphic value should not be allowed as it breaks the abstraction of parametricity. Also, we cannot be sure that the actual type is something that has a sensible string representation (it could be a closure for example) unless we have some kind of bounded polymorphism (something like class Foo<t : Showable> could be used to guarantee that t has a toString method).

[EliasC]

To put an end to this, I suggest checking the arguments of print for polymorphic values to reject such programs.

[PhucVH888]

I found the typeToPrintfFstr definition in Expr.hs but I don't quite understand how it works.

typeToPrintfFstr :: Ty.Type -> String
typeToPrintfFstr ty
    | Ty.isIntType ty          = "%lli"
    | Ty.isRealType ty         = "%f"
    | Ty.isStringObjectType ty = "%s"
    | Ty.isStringType ty       = "%s"
    | Ty.isCharType ty         = "%c"
    | Ty.isBoolType ty         = "bool<%zd>"
    | Ty.isRefType ty          = show ty ++ "<%p>"
    | Ty.isFutureType ty       = "fut<%p>"
    | otherwise = case translate ty of
                    Ptr something -> "%p"
                    _ -> error $ "Expr.hs: typeToPrintfFstr not defined for " ++ show ty

and in typechecker.hs

    --  count("{}", stringLit) = n
    --  E |- arg1 : t1 .. E |- argn : tn
    -- ---------------------------------------------
    --  E |- print(stringLit, arg1 .. argn) : void
    doTypecheck e@(Print {args = []}) = do
        let meta = emeta e
            eFormatString = setType stringType $ StringLiteral meta "\n"
        return $ setType voidType e {args = [eFormatString]}

    doTypecheck e@(Print {args = [arg]}) =
        do eArg <- doTypecheck arg
           let meta = emeta e
               eFormatString = setType stringType $ StringLiteral meta "{}\n"
           let newArgs = [eFormatString, eArg]
           return $ setType voidType e {args = newArgs}

    doTypecheck e@(Print {args}) =
        do eArgs <- mapM typecheck args
           let meta = emeta e
               fst = head eArgs
               rest = tail eArgs
               sugaredFst = fromJust $ getSugared fst
           unless (isStringLiteral sugaredFst) $
                  tcError $ "Formatted printing expects first argument '" ++
                            show (ppSugared fst) ++ "' to be a string literal"
           let formatString = stringLit sugaredFst
               noArgs = T.count (T.pack "{}") (T.pack formatString)
           unless (noArgs == length rest) $
                  tcError $ "Wrong number of arguments to format string. " ++
                            "Expected " ++ show noArgs ++ ", got " ++
                            show (length rest) ++ "."
           let eFormatString = setType stringType $
                               StringLiteral meta formatString
               newArgs = eFormatString : rest
           return $ setType voidType e {args = newArgs}

[EliasC]

@PhucVH888: Your first snippet is part of the backend (that generates C-code from Encore code) and it finds the correct format specifier for the resulting printf call. For example, printing an print 42 is more or less translated into printf("%lli\n", 42).

Your second snippet is the type checking code for the Print expression. The three cases are for when there are no arguments, one argument, and more than one argument respectively. If you clarify which parts you don't understand, maybe I can help!

[TobiasWrigstad]

@EliasC Did you implement this fix? What is the status on this?

[EliasC]

Fixed (by disallowing printing of polymorphic values) by #380