How to create ADT in Haskell?
In Scala I can describe such ADT:
sealed trait Foo
case class A(a: Int) extends Foo
case class B(b: String) extends Foo
case class C(a: A, b: B) extends Foo
How can I do the same in Haskell?
data Foo = A Int | B String | C A B
It doesn't work, because A and B are not types. Should I use GHC extensions to do it?
scala haskell algebraic-data-types
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In Scala I can describe such ADT:
sealed trait Foo
case class A(a: Int) extends Foo
case class B(b: String) extends Foo
case class C(a: A, b: B) extends Foo
How can I do the same in Haskell?
data Foo = A Int | B String | C A B
It doesn't work, because A and B are not types. Should I use GHC extensions to do it?
scala haskell algebraic-data-types
edited 2 days ago
mfirry
2,75811729
asked 2 days ago
AndrewAndrew
834
New contributor
Andrew is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
add a comment |
In Scala I can describe such ADT:
sealed trait Foo
case class A(a: Int) extends Foo
case class B(b: String) extends Foo
case class C(a: A, b: B) extends Foo
How can I do the same in Haskell?
data Foo = A Int | B String | C A B
It doesn't work, because A and B are not types. Should I use GHC extensions to do it?
scala haskell algebraic-data-types
edited 2 days ago
mfirry
2,75811729
asked 2 days ago
AndrewAndrew
834
New contributor
Andrew is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
In Scala I can describe such ADT:
sealed trait Foo
case class A(a: Int) extends Foo
case class B(b: String) extends Foo
case class C(a: A, b: B) extends Foo
How can I do the same in Haskell?
data Foo = A Int | B String | C A B
It doesn't work, because A and B are not types. Should I use GHC extensions to do it?
scala haskell algebraic-data-types
scala haskell algebraic-data-types
edited 2 days ago
mfirry
2,75811729
asked 2 days ago
AndrewAndrew
834
New contributor
Andrew is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
edited 2 days ago
mfirry
2,75811729
asked 2 days ago
AndrewAndrew
834
New contributor
Andrew is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
edited 2 days ago
mfirry
2,75811729
edited 2 days ago
mfirry
2,75811729
edited 2 days ago
mfirry
2,75811729
2,75811729
asked 2 days ago
AndrewAndrew
834
New contributor
Andrew is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
asked 2 days ago
AndrewAndrew
834
asked 2 days ago
AndrewAndrew
834
834
New contributor
Andrew is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
New contributor
Andrew is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
Andrew is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
add a comment |
add a comment |
1 Answer
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In Scala, your ADT makes A,B,C to be subtypes of Foo. In Haskell we do not have subtypes, so A,B,C are instead constructors of type Foo.
A few possible workarounds:
Repeat the fields. This is the most basic option.
data Foo = A Int | B String | C Int String
Define additional types, so that we can reuse them more than once.
data AT = AT Int -- can have many arguments
data BT = BT String -- can have many arguments
data Foo = A AT | B BT | C AT BT
Exploit a GADT
data FooTag = AT | BT | CT
data Foo (tag :: FooTag) where
A :: Int -> Foo 'AT
B :: String -> Foo 'BT
C :: Foo 'AT -> Foo 'BT -> Foo 'CT
Here, in the last line we are able to refer to "the values constructed using
A" using the typeFoo 'AT, since tagATis only used by constructorA.
Note that this approach adds a tag parameter toFoo, so it slightly changes the interface: we can no longer writebar :: Foo -> ..., but we have to writebar :: Foo t -> ...(or to use existential types).
answered 2 days ago
chichi
76.9k287145
4
The last example requires several extensions, though, right? (DataKinds,GADTs, andKindSignatures, I think.)
– chepner
2 days ago
add a comment |
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1 Answer
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1 Answer
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active
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active
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In Scala, your ADT makes A,B,C to be subtypes of Foo. In Haskell we do not have subtypes, so A,B,C are instead constructors of type Foo.
A few possible workarounds:
Repeat the fields. This is the most basic option.
data Foo = A Int | B String | C Int String
Define additional types, so that we can reuse them more than once.
data AT = AT Int -- can have many arguments
data BT = BT String -- can have many arguments
data Foo = A AT | B BT | C AT BT
Exploit a GADT
data FooTag = AT | BT | CT
data Foo (tag :: FooTag) where
A :: Int -> Foo 'AT
B :: String -> Foo 'BT
C :: Foo 'AT -> Foo 'BT -> Foo 'CT
Here, in the last line we are able to refer to "the values constructed using
A" using the typeFoo 'AT, since tagATis only used by constructorA.
Note that this approach adds a tag parameter toFoo, so it slightly changes the interface: we can no longer writebar :: Foo -> ..., but we have to writebar :: Foo t -> ...(or to use existential types).
answered 2 days ago
chichi
76.9k287145
4
The last example requires several extensions, though, right? (DataKinds,GADTs, andKindSignatures, I think.)
– chepner
2 days ago
add a comment |
In Scala, your ADT makes A,B,C to be subtypes of Foo. In Haskell we do not have subtypes, so A,B,C are instead constructors of type Foo.
A few possible workarounds:
Repeat the fields. This is the most basic option.
data Foo = A Int | B String | C Int String
Define additional types, so that we can reuse them more than once.
data AT = AT Int -- can have many arguments
data BT = BT String -- can have many arguments
data Foo = A AT | B BT | C AT BT
Exploit a GADT
data FooTag = AT | BT | CT
data Foo (tag :: FooTag) where
A :: Int -> Foo 'AT
B :: String -> Foo 'BT
C :: Foo 'AT -> Foo 'BT -> Foo 'CT
Here, in the last line we are able to refer to "the values constructed using
A" using the typeFoo 'AT, since tagATis only used by constructorA.
Note that this approach adds a tag parameter toFoo, so it slightly changes the interface: we can no longer writebar :: Foo -> ..., but we have to writebar :: Foo t -> ...(or to use existential types).
answered 2 days ago
chichi
76.9k287145
4
The last example requires several extensions, though, right? (DataKinds,GADTs, andKindSignatures, I think.)
– chepner
2 days ago
add a comment |
In Scala, your ADT makes A,B,C to be subtypes of Foo. In Haskell we do not have subtypes, so A,B,C are instead constructors of type Foo.
A few possible workarounds:
Repeat the fields. This is the most basic option.
data Foo = A Int | B String | C Int String
Define additional types, so that we can reuse them more than once.
data AT = AT Int -- can have many arguments
data BT = BT String -- can have many arguments
data Foo = A AT | B BT | C AT BT
Exploit a GADT
data FooTag = AT | BT | CT
data Foo (tag :: FooTag) where
A :: Int -> Foo 'AT
B :: String -> Foo 'BT
C :: Foo 'AT -> Foo 'BT -> Foo 'CT
Here, in the last line we are able to refer to "the values constructed using
A" using the typeFoo 'AT, since tagATis only used by constructorA.
Note that this approach adds a tag parameter toFoo, so it slightly changes the interface: we can no longer writebar :: Foo -> ..., but we have to writebar :: Foo t -> ...(or to use existential types).
answered 2 days ago
chichi
76.9k287145
In Scala, your ADT makes A,B,C to be subtypes of Foo. In Haskell we do not have subtypes, so A,B,C are instead constructors of type Foo.
A few possible workarounds:
Repeat the fields. This is the most basic option.
data Foo = A Int | B String | C Int String
Define additional types, so that we can reuse them more than once.
data AT = AT Int -- can have many arguments
data BT = BT String -- can have many arguments
data Foo = A AT | B BT | C AT BT
Exploit a GADT
data FooTag = AT | BT | CT
data Foo (tag :: FooTag) where
A :: Int -> Foo 'AT
B :: String -> Foo 'BT
C :: Foo 'AT -> Foo 'BT -> Foo 'CT
Here, in the last line we are able to refer to "the values constructed using
A" using the typeFoo 'AT, since tagATis only used by constructorA.
Note that this approach adds a tag parameter toFoo, so it slightly changes the interface: we can no longer writebar :: Foo -> ..., but we have to writebar :: Foo t -> ...(or to use existential types).
answered 2 days ago
chichi
76.9k287145
edited 2 days ago
answered 2 days ago
chichi
76.9k287145
answered 2 days ago
chichi
76.9k287145
answered 2 days ago
chichi
76.9k287145
76.9k287145
4
The last example requires several extensions, though, right? (DataKinds,GADTs, andKindSignatures, I think.)
– chepner
2 days ago
add a comment |
4
The last example requires several extensions, though, right? (DataKinds,GADTs, andKindSignatures, I think.)
– chepner
2 days ago
4
4
The last example requires several extensions, though, right? (
DataKinds, GADTs, and KindSignatures, I think.)– chepner
2 days ago
The last example requires several extensions, though, right? (
DataKinds, GADTs, and KindSignatures, I think.)– chepner
2 days ago
add a comment |
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