[Haskell-beginners] Monad question

[יהושע ולך]

convert (Var mx) = mx >>= (return .Var x)

Should also be the non-do-notation.
(again, typing from a phone, and not tested)

On Thu, Nov 25, 2021, 19:46 יהושע ולך <yehoshuapw at gmail.com> wrote:

> 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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>
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