This tag concerns pattern match coverage errors or warnings in functional programming languages with algebraic data types and pattern matching. Questions with this tag can be about why a compiler detects missing patterns or why missing patterns cause an error at run time.
Questions tagged [non-exhaustive-patterns]
85 questions
1
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2 answers
How to fix "Non-exhaustive patterns in function"
I want pass a list as parameter to a function which multiply every element of this list by 3. I has to use recursion (I know how to do it) and map function (there is a problem).
I'm trying passing a list as a parameter as I've seen in other posts…
Barburka
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1
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1 answer
"Non-exhaustive patterns in function" error when appending value before function call
I'm not sure what I'm not handling. Suppose I have a function, that converts an integer to a string. Call it converter.
Now, to convert position integer to string, I just call converter. To convert a negative integer to string, I append - to the…
SilentDev
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1 answer
How to implement a function to change array cells avoiding non-exhaustive pattern errors in haskell?
My goal is to write a function called walk, which switches the value of a labyrinth cell with its neighbour.
For example calling walk 0 labyrinthA should move the T one cell to the left.Here i tried to change a value of a labyrinth cell in…
that-cs-dude
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1
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0 answers
Non-exhaustive patterns error when defining foldr variant
I was asked to make a function that works like foldr but with non empty lists, that works like this: foldr1 f [x1,x2...xn] = f x1 (f x2...(f xn-1 xn)...).
So I defined it like this:
foldr1 f [x] = x
foldr1 f (x:xs) = f x (foldr1 f xs)
foldr1 f _ =…
Jaime Fernández
- 131
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1
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2 answers
How to deal with recursing through a list in Haskell (transpose operation)
While I understand that there may be transpose or ZipList functions in Haskell, I am trying to build my own transpose function that will take n lists of equal length m and transpose them into m lists of length n.
So far I have the function nearly…
Ron F
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2 answers
Non-exhaustive patterns, Haskell
I am trying to write the function tails, which converts a string into a list of strings in the following way:
tails "abc" = ["abc", "bc", "c", ""]
Here is my implementation:
tails :: [Char] -> [[Char]]
tails (x:xs)
| length (x:xs) == 0 = [""]
…
user376127
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1
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1 answer
Match nonexhaustive im ML treemap function
Hello I am new to ML and am writing a treemap function for the following datatype :
datatype tree = NIL | CONS of (tree * tree) | LEAF of int;
This is my treemap function:
fun treemap f = fn LEAF x => LEAF (f x)
| CONS(y,z) => CONS…
user3530192
- 31
- 4
1
vote
1 answer
Exhaustiveness check in typeclasses for promoted types
I have promoted type Nat = Suc Nat | Zero and I want to make a typeclass class C (a :: Nat) b. Is there a way to convince GHC that instance C Zero b and instance C (Seq x) b covers all cases and therefore I don't need to explicitly declare C as a…
fakedrake
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Haskell List Comprehension Non-exhaustive pattern when calling more than one parameter
To start with I have created a Type StudentMark which is a tuple taking firstly a String and secondly an Int.
type StudentMark = (String, Int)
This is my capMarks function:
capMarks :: [StudentMark] -> [StudentMark]
capMarks [cMarks] = [(st, mk) |…
GarethAS
- 331
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1
vote
1 answer
How to detect the end of a list in Haskell?
I'm writting a recursive function that use specific formulas to calculate 2 lists.
But I will simplify the function so you can understand the problem I'm having because the point here is to detect [] of the list.
So I've the following…
Lord Rixuel
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1 answer
Haskell: Non-exhaustive pattern - Checking if list is ascending
I have no idea why my function doesn't work. I have gone through all the posts about non-exhaustive functions but my functions fulfills all possible options as far as I can see.
ascending :: [Int] -> Bool
ascending [] = error "Empty…
deadfire19
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1
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2 answers
Non-exhaustive patterns in function haskell
I think I am missing the case where there's a one element list but I can't find a way to write it can someone help me?
getBoard :: [String] -> [String]
getBoard (h:t) | isLOL h = h: getBoard (t)
| otherwise = []
isLOL :: String ->…
45Yoda
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1
vote
1 answer
Haskell Non-Exhaustion
I was wondering if anyone could help me identify the part of the haskell code that would be non-exhaustive? I can't see how the base case isn't met at the end of the list.
thanks a lot
Jonny
type Rule
= (Char, String)
type Rules
= [Rule]
type…
FoxGlove
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1
vote
0 answers
Creating an infix operator
I've run into a little problem i can't seem to solve, been at it for a few hours now to no avail.
I've defined a datatype, Hand.
data Hand = Empty | Add Card Hand
deriving (Eq, Show)
And another type i'm using:
data Card = Card { rank ::…
Rewbert
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1 answer
Haskell - Non-exhaustive patterns in case
I have got the following code:
F (S core ps) = FAll core [] ps
where
FAll core acc ((name, (pc : pcs)) : ps)
= case F' (pc : pcs) (readC pc core) core of
Nothing ->
if (length pcs) /= 0 then FAll…
user1054204