I can't actually cheok right now,

but 

```

convert (Var mv) = do

    v <- mv

     return $ Var v

```

is the "do notation" which should work.

(and similarly for the other expression)

On Thu, Nov 25, 2021, 19:13 mike h <mike_k_houghton@yahoo.co.uk> wrote:

Hi,
This isn’t homework! I’ve been staring at this for several hours - and that usually works.
I also tried typed holes to no avail.  I thinks it is  quite simple really but I’ve gone past seeing it!

I have
data Expr a = Var a | Add (Expr a) (Expr a)

and would like to write

convert :: Expr (Maybe a) -> Maybe (Expr a)

which returns Nothing if there is an occurrence of Nothing inside the
input expression e, otherwise it returns Just e', where e'
is a new expression where the internal values of type a are not wrapped in Just.
You should use the functionality of the Maybe monad to implement
the convert function.


Thanks
Mike
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