refutable patterns require MonadFail

[jwaldmann]

(with ghc-9.0.1-alpha1) when compiling

model :: ExampleModel ()
model _ = exists $ \col -> do
  [x1,x2,x3,x4,x5,x6,x7,x8,x9,x10] <- colList col 10
  ...

I am seeing

examples/Olympic.hs:40:3: error:
    • Could not deduce (MonadFail m)
        arising from a do statement 
        with the failable pattern ‘[x1,  ...
      from the context: (Show (Control.CP.FD.FD.FDIntTerm s), FDSolver s,
                         Control.CP.SearchTree.MonadTree m,
                         Control.CP.SearchTree.TreeSolver m ~ FDInstance s)
...

the suggested fix seems impossible (there is no m)

      Possible fix:
        add (MonadFail m) to the context of
          the type signature for:
            model :: ExampleModel ()

But still, someone needs to get a MonadFail instance.

[jwaldmann]

I will do

class (MonadFail m, Solver (TreeSolver m)) => MonadTree m 
  ...
   exists :: ...

instance Solver solver => MonadFail (Tree solver)
  fail = error

then a failed match will produce

olympic: Pattern match failure in do expression at examples/Olympic.hs:40:3-36
CallStack (from HasCallStack):
  error, called at src/Control/CP/SearchTree.hs:198:10 in monadiccp-0.7.7-inplace:Control.CP.SearchTree