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
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 _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners