[Haskell-beginners] Monad question

[יהושע ולך]

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 at 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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