All Questions
Tagged with option-type haskell
247
questions
5
votes
1
answer
95
views
In Frege how can I convert an String safely to a Maybe Int?
In Frege I want to convert an String to an Int, but need to handle unparsable strings too.
So I guess I'm looking for something like a readMaybe function. Where do I find this? Or how can I use Java's ...
- 9,924
5
votes
4
answers
249
views
In Functional Programming, why the Maybe functor is not defined as data Maybe a = Nothing | a?
I was wondering why in Haskell (and FP in general) functors such as the Maybe functor are defined as
data Maybe a = Nothing | Just a
Instead of simply
data Maybe a = Nothing | a
The second option ...
- 1,629
1
vote
0
answers
89
views
Own mapMaybe does not parse
So I have the following type annotation in Haskell:
mapMaybe :: (a -> b) -> Maybe a -> Maybe b
And it is defined as follows:
mapMaybe _ Nothing = Nothing
mapMaybe f (Just x) = Just (head (...
- 929
1
vote
3
answers
132
views
Accumulate a Function Over a List of Monads (Maybe)
I'm learning Haskell as a way of getting to grips with a new programming paradigm. I'm getting hung up on a specific, probably basic, issue. While I'm finding lots of documentation and other stack ...
- 2,184
0
votes
1
answer
103
views
Wrap a partial pattern matching into a Maybe
Is there any way to wrap a partial pattern-match into a Maybe?
I wish to write some code kind of like this:
fromMaybe defaultValue $ do
[a, b] <- mkList :: [Int]
[x] <- mkList' a b :: [Int]
...
- 1,026
2
votes
1
answer
199
views
Inverted [Maybe a] tranverse behavior
I want to create a function that receives some items to process, returning Nothing when none of the items could be processed and Just if at least one item was processed.
To me, this sounds a lot like ...
- 2,068
1
vote
2
answers
270
views
Are Maybe String and Maybe Int both Monoids in Haskell?
I´ve gotten into a bit of confusion whether Maybe Int and Maybe String are both monoids in Haskell because i dont really know what functions you can apply to Int or String in terms of monoids with ...
- 29
4
votes
2
answers
248
views
Combine two Just values if present, else return first Just
I want a shorter way to write this block of code as this is going to be used in multiple places in a project I am working on. Is there a library I can use which would have a function for my specific ...
- 41
2
votes
1
answer
516
views
What is the advantage of Option/Maybe Monad over Functor?
I understand the advantage of IO monad and List Monad over Functor, however, I don't understand the advantage of Option/Maybe Monad over Functor.
Is that simply the integration of types of the ...
- 642
1
vote
1
answer
149
views
Non-exhaustive patterns in function - Maybe type [duplicate]
In the following function definition I am getting Non-exhaustive patterns in function tapChar.
What am I missing?
reverseTap :: DaPhone -> Char -> [(Digit, Presses)]
reverseTap phone s
| ...
- 39
4
votes
1
answer
457
views
How can I handle Nothing branch of maybe monad gracefully?
data Union = F Float | I Int | S String
getUnionFromString :: String -> Union
getUnionFromString str =
case (readMaybe str :: Maybe Int) of
Just i -> I i
Nothing ->
...
- 459
4
votes
4
answers
788
views
How to recursively iterate over a list with Maybe objects in Haskell
I am trying to create a function justifyList that takes a list of Maybe objects ([Maybe a]) and return a list of all objects of Just type, discarding any element that is Nothing in the parameter list.
...
- 782
0
votes
1
answer
423
views
Finding max of a list of Maybe values in Haskell using Higher Order Functions
I am trying to use higher level order functions to take a list of Maybe values and return a max of type int but for some reason I can only return a Maybe value.
For example this works:
max' iL = foldl ...
- 3
0
votes
1
answer
139
views
What is This More General Almost-Traversal Operation
Trying to wrap my mind around traversals - or maybe something slightly different, in this case.
I understand a traversal to be an operation that performs an applicative operation (or effect) over a ...
- 3,041
-1
votes
1
answer
148
views
How to stop Maybe from being passed into another function in Haskell [closed]
I have already looked at this answer, but it is unclear to me how to apply it in my situation.
I have the following function:
designC :: DesignC -> Maybe CDef -> String
designC FirstString _ = &...
- 23