Question about Pattern match(es) are non-exhaustive error

Questions
1

G-d willing

Hello,
Following to my previous post, I am using the --ghc-option=-Wincomplete-uni-patterns in my build-options
This time I am getting an error on the following code:

testCheckConstraintsAgainst : Script ()
testCheckConstraintsAgainst = do
  let record1 = Record 2 True (-1)
      record2 = Record (-1) False 2
      record3 = Record 1 True 0
      Right b = typecheckAgainst @Record $ var "record.foo" `mul` 1
      Right b' = typecheckAgainst @Record $ var "record.foo" `minus` 1

The error message is:

Pattern match(es) are non-exhaustive
  In a pattern binding: Patterns not matched: (Left _)

Since I am new to Daml, I know to use Right and Left on a case switch, but I am not sure how it works when I just type it inside a let command.

2

I think your best option here is to do a case on the right

let b = case typecheckAgainst @Record $ var "record.foo" `mul` 1 of
          Right b -> b
          Left e -> doSomething
3

Thanks for the reply but this is still not good enough.
Keep in mind that I am using the Wincomplete-uni-patterns build option.
Basically after the code I attached, there is usage for b & b’. So in that case b is not defined anymore as the right value, but the Either. And if I try to fix it by using Right whenever we use it, I am getting that error message again (Pattern match(es) are non-exhaustive In a pattern binding: Patterns not matched: (Left _)
So, I am not sure how to get rid of that loop.

4

What do you want to happen in the Left case? Can you share the code afterwards that uses it?

5

Well the other 2 lines after that are:

      dynamicConstraint = mkConstraintUnsafe @Record (var "record.b" `gt` 0) "b is positive" [mkBinding "b" b]
      updatedConstraint = updateConstraintBindingsUnsafe dynamicConstraint [mkBinding "b" b']
6

Can you share what you expect to happen if you get a Left?

7

Well, I was offered a solution which worked well for me at this point and it was adding the following code:

testTypedExpr : Either TypecheckErr (TypedExprAgainst a) -> (TypedExprAgainst a)
testTypedExpr val = case val of
  Right result -> result
  Left _ -> error "Error"

and the previous 2 lines I changed to:

b = testTypedExpr $ typecheckAgainst @Record $ var "record.foo" `mul` 1
      b' = testTypedExpr $ typecheckAgainst @Record $ var "record.foo" `minus` 1
8

Isn’t that exactly the case solution I suggested? I think I don’t understand why that did not work for you.

9

G-d willing

No it is not the same since using the case will return Either value and not the Right value. And since I am using the returned value later on, it won’t work.
That will force me again to use the Right and Left on the same returned value - so it is kind of an infinite loop.

10

I don’t think that should be true. If you look at my example, note that I also extract the value from the Either

 Right b -> b

What you say would apply if we had

 Right b -> Right b
11

If you have a concrete example and an error message, I’m happy to take a look.