Calling a function with user defined datatypes in Haskell

up vote
1
down vote

favorite

I defined a data type in Haskell

data List a=Nil

 |Cons a (List a)

I wrote a function using this data type

listLength Nil=0

listLength (Cons x xs)=1+listLength(xs)

I tried to call that function giving arguments like this

listLength (Cons 2 [2,3])

But I got an error:

<interactive>:68:20: error:

    * Couldn't match expected type `List Integer'

                  with actual type `[Integer]'

    * In the second argument of `Cons', namely `[2, 3]'

      In the first argument of `listLength', namely `(Cons 2 [2, 3])'

      In the expression: listLength (Cons 2 [2, 3])

How do call this function?

asked Nov 11 at 6:37

Nishara Kavindi

268

4

Try listLength (Cons 2 (Cons 2 Nil)). The issue is that [2,3] has type [Integer], but the Cons constructor requires a second argument of type List Integer, and due to Haskell's strong typing the two types are not the same!
– bradrn
Nov 11 at 6:41

It worked. Thank you
– Nishara Kavindi
Nov 11 at 6:49

2

To ease testing you can define and use some auxiliary conversion function like fromList = foldr Cons Nil and then write listLength (fromList [1,7,2,5])). In this way you can convert standard lists to your lists before testing, and avoid to type all the Conses.
– chi
Nov 11 at 8:12

1

@Nishara Kavindi Do you want me to turn my comment into a proper answer so this question can be marked as 'answered'?
– bradrn
Nov 11 at 8:36

@bradrn Okey.You can do that.
– Nishara Kavindi
Nov 12 at 16:52

add a comment |

up vote
1
down vote

favorite

I defined a data type in Haskell

data List a=Nil

 |Cons a (List a)

I wrote a function using this data type

listLength Nil=0

listLength (Cons x xs)=1+listLength(xs)

I tried to call that function giving arguments like this

listLength (Cons 2 [2,3])

But I got an error:

<interactive>:68:20: error:

    * Couldn't match expected type `List Integer'

                  with actual type `[Integer]'

    * In the second argument of `Cons', namely `[2, 3]'

      In the first argument of `listLength', namely `(Cons 2 [2, 3])'

      In the expression: listLength (Cons 2 [2, 3])

How do call this function?

asked Nov 11 at 6:37

Nishara Kavindi

268

4

Try listLength (Cons 2 (Cons 2 Nil)). The issue is that [2,3] has type [Integer], but the Cons constructor requires a second argument of type List Integer, and due to Haskell's strong typing the two types are not the same!
– bradrn
Nov 11 at 6:41

It worked. Thank you
– Nishara Kavindi
Nov 11 at 6:49

2

To ease testing you can define and use some auxiliary conversion function like fromList = foldr Cons Nil and then write listLength (fromList [1,7,2,5])). In this way you can convert standard lists to your lists before testing, and avoid to type all the Conses.
– chi
Nov 11 at 8:12

1

@Nishara Kavindi Do you want me to turn my comment into a proper answer so this question can be marked as 'answered'?
– bradrn
Nov 11 at 8:36

@bradrn Okey.You can do that.
– Nishara Kavindi
Nov 12 at 16:52

add a comment |

up vote
1
down vote

favorite

I defined a data type in Haskell

data List a=Nil

 |Cons a (List a)

I wrote a function using this data type

listLength Nil=0

listLength (Cons x xs)=1+listLength(xs)

I tried to call that function giving arguments like this

listLength (Cons 2 [2,3])

But I got an error:

<interactive>:68:20: error:

    * Couldn't match expected type `List Integer'

                  with actual type `[Integer]'

    * In the second argument of `Cons', namely `[2, 3]'

      In the first argument of `listLength', namely `(Cons 2 [2, 3])'

      In the expression: listLength (Cons 2 [2, 3])

How do call this function?

asked Nov 11 at 6:37

Nishara Kavindi

268

I defined a data type in Haskell

data List a=Nil

 |Cons a (List a)

I wrote a function using this data type

listLength Nil=0

listLength (Cons x xs)=1+listLength(xs)

I tried to call that function giving arguments like this

listLength (Cons 2 [2,3])

But I got an error:

<interactive>:68:20: error:

    * Couldn't match expected type `List Integer'

                  with actual type `[Integer]'

    * In the second argument of `Cons', namely `[2, 3]'

      In the first argument of `listLength', namely `(Cons 2 [2, 3])'

      In the expression: listLength (Cons 2 [2, 3])

How do call this function?

haskell

asked Nov 11 at 6:37

Nishara Kavindi

268

asked Nov 11 at 6:37

Nishara Kavindi

268

asked Nov 11 at 6:37

Nishara Kavindi

268

asked Nov 11 at 6:37

Nishara Kavindi

268

asked Nov 11 at 6:37

Nishara Kavindi

268

4

Try listLength (Cons 2 (Cons 2 Nil)). The issue is that [2,3] has type [Integer], but the Cons constructor requires a second argument of type List Integer, and due to Haskell's strong typing the two types are not the same!
– bradrn
Nov 11 at 6:41

It worked. Thank you
– Nishara Kavindi
Nov 11 at 6:49

2

To ease testing you can define and use some auxiliary conversion function like fromList = foldr Cons Nil and then write listLength (fromList [1,7,2,5])). In this way you can convert standard lists to your lists before testing, and avoid to type all the Conses.
– chi
Nov 11 at 8:12

1

@Nishara Kavindi Do you want me to turn my comment into a proper answer so this question can be marked as 'answered'?
– bradrn
Nov 11 at 8:36

@bradrn Okey.You can do that.
– Nishara Kavindi
Nov 12 at 16:52

add a comment |

4

Try listLength (Cons 2 (Cons 2 Nil)). The issue is that [2,3] has type [Integer], but the Cons constructor requires a second argument of type List Integer, and due to Haskell's strong typing the two types are not the same!
– bradrn
Nov 11 at 6:41

It worked. Thank you
– Nishara Kavindi
Nov 11 at 6:49

2

To ease testing you can define and use some auxiliary conversion function like fromList = foldr Cons Nil and then write listLength (fromList [1,7,2,5])). In this way you can convert standard lists to your lists before testing, and avoid to type all the Conses.
– chi
Nov 11 at 8:12

1

@Nishara Kavindi Do you want me to turn my comment into a proper answer so this question can be marked as 'answered'?
– bradrn
Nov 11 at 8:36

@bradrn Okey.You can do that.
– Nishara Kavindi
Nov 12 at 16:52

Try listLength (Cons 2 (Cons 2 Nil)). The issue is that [2,3] has type [Integer], but the Cons constructor requires a second argument of type List Integer, and due to Haskell's strong typing the two types are not the same!
– bradrn
Nov 11 at 6:41

It worked. Thank you
– Nishara Kavindi
Nov 11 at 6:49

To ease testing you can define and use some auxiliary conversion function like fromList = foldr Cons Nil and then write listLength (fromList [1,7,2,5])). In this way you can convert standard lists to your lists before testing, and avoid to type all the Conses.
– chi
Nov 11 at 8:12

@Nishara Kavindi Do you want me to turn my comment into a proper answer so this question can be marked as 'answered'?
– bradrn
Nov 11 at 8:36

@bradrn Okey.You can do that.
– Nishara Kavindi
Nov 12 at 16:52

add a comment |

1 Answer
1

active

oldest

votes

up vote
0
down vote

accepted

From my comment above:

Try using listLength (Cons 2 (Cons 2 Nil)). The issue is that [2,3] has type [Integer], but the Cons constructor requires a second argument of type List Integer, and due to Haskell's strong typing the two types are not the same!

answered Nov 13 at 0:12

bradrn

18529

add a comment |

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1 Answer
1

active

oldest

votes

1 Answer
1

active

oldest

votes

up vote
0
down vote

accepted

From my comment above:

Try using listLength (Cons 2 (Cons 2 Nil)). The issue is that [2,3] has type [Integer], but the Cons constructor requires a second argument of type List Integer, and due to Haskell's strong typing the two types are not the same!

answered Nov 13 at 0:12

bradrn

18529

add a comment |

up vote
0
down vote

accepted

From my comment above:

Try using listLength (Cons 2 (Cons 2 Nil)). The issue is that [2,3] has type [Integer], but the Cons constructor requires a second argument of type List Integer, and due to Haskell's strong typing the two types are not the same!

answered Nov 13 at 0:12

bradrn

18529

add a comment |

up vote
0
down vote

accepted

From my comment above:

Try using listLength (Cons 2 (Cons 2 Nil)). The issue is that [2,3] has type [Integer], but the Cons constructor requires a second argument of type List Integer, and due to Haskell's strong typing the two types are not the same!

answered Nov 13 at 0:12

bradrn

18529

From my comment above:

Try using listLength (Cons 2 (Cons 2 Nil)). The issue is that [2,3] has type [Integer], but the Cons constructor requires a second argument of type List Integer, and due to Haskell's strong typing the two types are not the same!

answered Nov 13 at 0:12

bradrn

18529

answered Nov 13 at 0:12

bradrn

18529

answered Nov 13 at 0:12

bradrn

18529

answered Nov 13 at 0:12

bradrn

18529

add a comment |

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