Calling a function with user defined datatypes in Haskell
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
add a comment |
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
4
TrylistLength (Cons 2 (Cons 2 Nil)). The issue is that[2,3]has type[Integer], but theConsconstructor requires a second argument of typeList 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 likefromList = foldr Cons Niland then writelistLength (fromList [1,7,2,5])). In this way you can convert standard lists to your lists before testing, and avoid to type all theConses.
– 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 |
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
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
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
268
4
TrylistLength (Cons 2 (Cons 2 Nil)). The issue is that[2,3]has type[Integer], but theConsconstructor requires a second argument of typeList 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 likefromList = foldr Cons Niland then writelistLength (fromList [1,7,2,5])). In this way you can convert standard lists to your lists before testing, and avoid to type all theConses.
– 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
TrylistLength (Cons 2 (Cons 2 Nil)). The issue is that[2,3]has type[Integer], but theConsconstructor requires a second argument of typeList 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 likefromList = foldr Cons Niland then writelistLength (fromList [1,7,2,5])). In this way you can convert standard lists to your lists before testing, and avoid to type all theConses.
– 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
4
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
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
It worked. Thank you
– Nishara Kavindi
Nov 11 at 6:49
2
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
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
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
@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
@bradrn Okey.You can do that.
– Nishara Kavindi
Nov 12 at 16:52
add a comment |
1 Answer
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From my comment above:
Try using
listLength (Cons 2 (Cons 2 Nil)). The issue is that[2,3]has type[Integer], but theConsconstructor requires a second argument of typeList Integer, and due to Haskell's strong typing the two types are not the same!
answered Nov 13 at 0:12
bradrn
385310
add a comment |
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1 Answer
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1 Answer
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oldest
votes
From my comment above:
Try using
listLength (Cons 2 (Cons 2 Nil)). The issue is that[2,3]has type[Integer], but theConsconstructor requires a second argument of typeList Integer, and due to Haskell's strong typing the two types are not the same!
answered Nov 13 at 0:12
bradrn
385310
add a comment |
From my comment above:
Try using
listLength (Cons 2 (Cons 2 Nil)). The issue is that[2,3]has type[Integer], but theConsconstructor requires a second argument of typeList Integer, and due to Haskell's strong typing the two types are not the same!
answered Nov 13 at 0:12
bradrn
385310
add a comment |
From my comment above:
Try using
listLength (Cons 2 (Cons 2 Nil)). The issue is that[2,3]has type[Integer], but theConsconstructor requires a second argument of typeList Integer, and due to Haskell's strong typing the two types are not the same!
answered Nov 13 at 0:12
bradrn
385310
From my comment above:
Try using
listLength (Cons 2 (Cons 2 Nil)). The issue is that[2,3]has type[Integer], but theConsconstructor requires a second argument of typeList Integer, and due to Haskell's strong typing the two types are not the same!
answered Nov 13 at 0:12
bradrn
385310
answered Nov 13 at 0:12
bradrn
385310
answered Nov 13 at 0:12
bradrn
385310
answered Nov 13 at 0:12
bradrn
385310
385310
add a comment |
add a comment |
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4
Try
listLength (Cons 2 (Cons 2 Nil)). The issue is that[2,3]has type[Integer], but theConsconstructor requires a second argument of typeList 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 Niland then writelistLength (fromList [1,7,2,5])). In this way you can convert standard lists to your lists before testing, and avoid to type all theConses.– 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