Abstraction of patterns in the pattern matching sense
In Haskell, use the PatternSynonyms language extension to turn on this feature (as of GHC 7.8+).
Questions tagged [pattern-synonyms]
48 questions
28
votes
5 answers
Haskell Bytestrings: How to pattern match?
I'm a Haskell newbie, and having a bit of trouble figuring out how to pattern match a ByteString. The [Char] version of my function looks like:
dropAB :: String -> String
dropAB [] = []
dropAB (x:[]) = x:[]
dropAB (x:y:xs) = if x=='a' &&…
LOS
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16
votes
1 answer
Pattern synonyms lead to unexhaustive pattern matching
I've managed to construct the following "minimal" example that shows my problem.
Provided the PatternSynonyms extension is enabled
data Vec = Vec Int Int
pattern Ve x y = Vec x y
f :: (Vec, Vec) -> Vec
f (v@(Ve a b), Ve c d)
| a > b = Vec…
Luka Horvat
- 4,283
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12
votes
2 answers
Pattern matching Data.Sequence like lists
I am using Data.Sequence instead lists for better performance. With lists we can do the following
foo :: [Int] -> Int
foo [] m = m
foo (x:xs) m = ...
How can this be accomplished with Data.Sequence. I have tried the following:
foo:: S.Seq Int ->…
abden003
- 1,325
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12
votes
1 answer
A list whose "Nil" carries a value?
Does some standard Haskell library define a data type like this
data ListWithEnd e a = Cons a (ListWithEnd e a)
| End e
That is a list whose terminating element carries a value of a designated type?
So ListWithEnd () is…
Petr
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11
votes
1 answer
Haskell: "Qualified name in binding position" error with Map.empty
I'm trying to create a pattern synonym for a newtype with an empty map.
{-# Language PatternSynonyms #-}
import qualified Data.Map as Map
newtype StoreEnv = StoreEnv (Map.Map Int String)
deriving (Eq, Show)
pattern EmptyStore ::…
Tien Ho
- 123
- 6
11
votes
2 answers
Pattern matching on a private data constructor
I'm writing a simple ADT for grid axis. In my application grid may be either regular (with constant step between coordinates), or irregular (otherwise). Of course, the regular grid is just a special case of irregular one, but it may worth to…
firegurafiku
- 3,017
- 1
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- 37
10
votes
1 answer
Combining patterns
Consider the following data type and pattern synonyms:
{-# LANGUAGE PatternSynonyms, NamedFieldPuns #-}
data Foo = Foo {
a :: Int
, b :: String
, c :: Maybe Bool
}
pattern Bar a b <- Foo { a, b }
pattern Baz c <- Foo { c }
I'd like…
wrl
- 131
- 4
8
votes
1 answer
Why does my pattern block an error on both sides?
To start off this whole thing I'm working with a pattern synonym defined as follows:
{-# Language PatternSynonyms #-}
pattern x := y <- x @ y
This allows me to run multiple pattern matches across a parameter at once. A regular as binding (@) does…
Wheat Wizard
- 3,982
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8
votes
1 answer
Pattern synonym can't unify types within type-level list
I'm getting an error when trying to define a pattern synonym based
on a GADT that has a type-level list.
I managed to boil it down to this example:
{-# LANGUAGE GADTs #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE…
rampion
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7
votes
1 answer
Writing a COMPLETE pragma for a polymorphic pattern synonym?
I have the following code and I don't know what should feed at ??. Or cannot polymorphic patterns make complete?
{-# LANGUAGE PatternSynonyms #-}
{-# LANGUAGE ViewPatterns #-}
module Data.Tuple.Single.Class
( Single (..)
, pattern Single
)…
Kazuki Okamoto
- 661
- 5
- 10
6
votes
3 answers
Haskell pattern matching on vectors
Is it possible to use list style pattern matching on vectors?
ie
import qualified Data.Vector as V
f :: V.Vector a -> a
f (x:xs) = x
gives an error
matt
- 1,817
- 14
- 35
6
votes
1 answer
How to Pattern Match an Empty Vector in Haskell?
Say I want to implement the length function for lists using pattern matching, then I could do something like this:
length' :: (Num b) => [a] -> b
length' [] = 0
length' (_:xs) = 1 + length' xs
Can I do something similar with Vectors?
frmsaul
- 1,366
- 2
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6
votes
1 answer
Why are all recursive pattern synonyms rejected?
{-# LANGUAGE PatternSynonyms, ViewPatterns #-}
data Quun = Foo | Bar | Oink Quun
fooey :: Quun -> Bool
fooey Foo = True
fooey (Oink Yum) = True
fooey _ = False
pattern Yum <- (fooey -> True)
This doesn't compile (at least in…
leftaroundabout
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5
votes
0 answers
Explicit type variable instantiation for pattern synonyms?
Suppose, I have the following code
data F a where
F :: Typeable a => F a
asType :: forall b a. Typeable b => F a -> Maybe (a :~: b, F b)
asType e@F{} = case eqT @b @a of
Just Refl -> Just (Refl, e)
Nothing -> Nothing
pattern AsType :: forall…
grepcake
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5
votes
1 answer
No-fun/puzzlement with pattern synonyms as functions
With PatternSynonyms (explicitly bidirectional form), the pattern-to-expr equations in effect form a function but spelled upper-case (providing you end up with an fully saturated data constr of the correct type). Then consider (at GHC 8.10.2)
{-#…
AntC
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