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?

edited Mar 23 at 13:21

mfirry

2,75811729

asked Mar 23 at 8:27

Andrew

834

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?

edited Mar 23 at 13:21

mfirry

2,75811729

asked Mar 23 at 8:27

Andrew

834

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?

edited Mar 23 at 13:21

mfirry

2,75811729

asked Mar 23 at 8:27

Andrew

834

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 Mar 23 at 13:21

mfirry

2,75811729

asked Mar 23 at 8:27

Andrew

834

edited Mar 23 at 13:21

mfirry

2,75811729

asked Mar 23 at 8:27

Andrew

834

edited Mar 23 at 13:21

mfirry

2,75811729

edited Mar 23 at 13:21

mfirry

2,75811729

edited Mar 23 at 13:21

mfirry

2,75811729

asked Mar 23 at 8:27

Andrew

834

asked Mar 23 at 8:27

Andrew

834

asked Mar 23 at 8:27

Andrew

834

add a comment |

1 Answer
1

active

oldest

votes

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 type Foo 'AT, since tag AT is only used by constructor A.
Note that this approach adds a tag parameter to Foo, so it slightly changes the interface: we can no longer write bar :: Foo -> ..., but we have to write bar :: Foo t -> ... (or to use existential types).

edited Mar 23 at 8:52

answered Mar 23 at 8:46

chi

77.2k287146

4

The last example requires several extensions, though, right? (DataKinds, GADTs, and KindSignatures, I think.)

– chepner
Mar 23 at 15:10

add a comment |

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1 Answer
1

active

oldest

votes

1 Answer
1

active

oldest

votes

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 type Foo 'AT, since tag AT is only used by constructor A.
Note that this approach adds a tag parameter to Foo, so it slightly changes the interface: we can no longer write bar :: Foo -> ..., but we have to write bar :: Foo t -> ... (or to use existential types).

edited Mar 23 at 8:52

answered Mar 23 at 8:46

chi

77.2k287146

4

The last example requires several extensions, though, right? (DataKinds, GADTs, and KindSignatures, I think.)

– chepner
Mar 23 at 15:10

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 type Foo 'AT, since tag AT is only used by constructor A.
Note that this approach adds a tag parameter to Foo, so it slightly changes the interface: we can no longer write bar :: Foo -> ..., but we have to write bar :: Foo t -> ... (or to use existential types).

edited Mar 23 at 8:52

answered Mar 23 at 8:46

chi

77.2k287146

4

The last example requires several extensions, though, right? (DataKinds, GADTs, and KindSignatures, I think.)

– chepner
Mar 23 at 15:10

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 type Foo 'AT, since tag AT is only used by constructor A.
Note that this approach adds a tag parameter to Foo, so it slightly changes the interface: we can no longer write bar :: Foo -> ..., but we have to write bar :: Foo t -> ... (or to use existential types).

edited Mar 23 at 8:52

answered Mar 23 at 8:46

chi

77.2k287146

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 type Foo 'AT, since tag AT is only used by constructor A.
Note that this approach adds a tag parameter to Foo, so it slightly changes the interface: we can no longer write bar :: Foo -> ..., but we have to write bar :: Foo t -> ... (or to use existential types).

edited Mar 23 at 8:52

answered Mar 23 at 8:46

chi

77.2k287146

edited Mar 23 at 8:52

answered Mar 23 at 8:46

chi

77.2k287146

answered Mar 23 at 8:46

chi

77.2k287146

answered Mar 23 at 8:46

chi

77.2k287146

4

The last example requires several extensions, though, right? (DataKinds, GADTs, and KindSignatures, I think.)

– chepner
Mar 23 at 15:10

add a comment |

4

The last example requires several extensions, though, right? (DataKinds, GADTs, and KindSignatures, I think.)

– chepner
Mar 23 at 15:10

The last example requires several extensions, though, right? (DataKinds, GADTs, and KindSignatures, I think.)

– chepner
Mar 23 at 15:10

add a comment |

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