Use 'any' to tell you true/false, if a list contains some value(s) divisible by 42












3














Having fun working through a Haskell tutorial...



One problem posed is as you see in the subject line... this is one I'd really like to figure out, but I'm at a loss... I've used any like so:



ghci >any (==55) [15,25,35,45,55,65,75,85,95]

True

ghci >any (==55) [15,25,35,45,54,65,75,85,95]

False

ghci >all even [2,4,6,8]

True

ghci >all even [1,3,5,7,9]

False


and it seems like checking if a list element is divisible by 42 or not, should be fairly easy...



I mean, you would check if any of the numbers in the list are n `mod` 42 == 0, right?



But how do you state that in the expression? Or do you need to write a 'helper' function?






haskell







share|improve this question




















  • 1




    Without a helper definition, the most straightforward way would be with an anonymous function (a "lambda").
    – duplode
    Nov 13 at 3:41










  • Holy cow, thank you everyone... i had actually thought of the lambda way later, but couldn't figure out putting together the function for some reason... but once i see it, I'm like, "that makes total sense".
    – Stormy
    Nov 14 at 6:14


















3














Having fun working through a Haskell tutorial...



One problem posed is as you see in the subject line... this is one I'd really like to figure out, but I'm at a loss... I've used any like so:



ghci >any (==55) [15,25,35,45,55,65,75,85,95]

True

ghci >any (==55) [15,25,35,45,54,65,75,85,95]

False

ghci >all even [2,4,6,8]

True

ghci >all even [1,3,5,7,9]

False


and it seems like checking if a list element is divisible by 42 or not, should be fairly easy...



I mean, you would check if any of the numbers in the list are n `mod` 42 == 0, right?



But how do you state that in the expression? Or do you need to write a 'helper' function?






haskell







share|improve this question




















  • 1




    Without a helper definition, the most straightforward way would be with an anonymous function (a "lambda").
    – duplode
    Nov 13 at 3:41










  • Holy cow, thank you everyone... i had actually thought of the lambda way later, but couldn't figure out putting together the function for some reason... but once i see it, I'm like, "that makes total sense".
    – Stormy
    Nov 14 at 6:14
















3












3








3







Having fun working through a Haskell tutorial...



One problem posed is as you see in the subject line... this is one I'd really like to figure out, but I'm at a loss... I've used any like so:



ghci >any (==55) [15,25,35,45,55,65,75,85,95]

True

ghci >any (==55) [15,25,35,45,54,65,75,85,95]

False

ghci >all even [2,4,6,8]

True

ghci >all even [1,3,5,7,9]

False


and it seems like checking if a list element is divisible by 42 or not, should be fairly easy...



I mean, you would check if any of the numbers in the list are n `mod` 42 == 0, right?



But how do you state that in the expression? Or do you need to write a 'helper' function?






haskell







share|improve this question















Having fun working through a Haskell tutorial...



One problem posed is as you see in the subject line... this is one I'd really like to figure out, but I'm at a loss... I've used any like so:



ghci >any (==55) [15,25,35,45,55,65,75,85,95]

True

ghci >any (==55) [15,25,35,45,54,65,75,85,95]

False

ghci >all even [2,4,6,8]

True

ghci >all even [1,3,5,7,9]

False


and it seems like checking if a list element is divisible by 42 or not, should be fairly easy...



I mean, you would check if any of the numbers in the list are n `mod` 42 == 0, right?



But how do you state that in the expression? Or do you need to write a 'helper' function?






haskell




haskell






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited Nov 13 at 3:39









duplode

22.8k44682




22.8k44682










asked Nov 13 at 3:35









Stormy

673




673








  • 1




    Without a helper definition, the most straightforward way would be with an anonymous function (a "lambda").
    – duplode
    Nov 13 at 3:41










  • Holy cow, thank you everyone... i had actually thought of the lambda way later, but couldn't figure out putting together the function for some reason... but once i see it, I'm like, "that makes total sense".
    – Stormy
    Nov 14 at 6:14
















  • 1




    Without a helper definition, the most straightforward way would be with an anonymous function (a "lambda").
    – duplode
    Nov 13 at 3:41










  • Holy cow, thank you everyone... i had actually thought of the lambda way later, but couldn't figure out putting together the function for some reason... but once i see it, I'm like, "that makes total sense".
    – Stormy
    Nov 14 at 6:14










1




1




Without a helper definition, the most straightforward way would be with an anonymous function (a "lambda").
– duplode
Nov 13 at 3:41




Without a helper definition, the most straightforward way would be with an anonymous function (a "lambda").
– duplode
Nov 13 at 3:41












Holy cow, thank you everyone... i had actually thought of the lambda way later, but couldn't figure out putting together the function for some reason... but once i see it, I'm like, "that makes total sense".
– Stormy
Nov 14 at 6:14






Holy cow, thank you everyone... i had actually thought of the lambda way later, but couldn't figure out putting together the function for some reason... but once i see it, I'm like, "that makes total sense".
– Stormy
Nov 14 at 6:14














2 Answers
2






active

oldest

votes


















6














Composing (0==) and (`mod 42`):



f :: [Integer] -> Bool

f = any ((0==).(`mod` 42))


futher reducing parenthesis noise:



f :: [Integer] -> Bool

f = any $ (0==).(`mod` 42)


Helper function:



f :: [Integer] -> Bool

f = any div42

  where

    div42 n = n `mod` 42 == 0


Stylistically, for this function, either way seems fine. However, say you wanted to check if any values are divisible by 42 OR 52, then utilizing composition may become more obfuscated/complex/futile. Whereas using a helper function keeps things readable: div42Or52 n = n `mod` 42 == 0 || n `mod` 52 == 0. Note, I've used a where clause above, but a let in expression or a lambda are possible alternative ways to structure helper functions.



Ultimately, its up to the developer to balance concise code with understandable code.






share|improve this answer



















  • 2




    For point-free code, it often helps readability to factor things into many small, generic, reusable definitions. For example, in this case I might write divisibleBy d = (== 0) . (`mod` d) (“to be divisible by d means to be equal to zero modulo d”) and then use any (divisibleBy 42). Nothing wrong with using a lambda or auxiliary definition in a where clause, though.
    – Jon Purdy
    Nov 13 at 8:17






  • 1




    @JonPurdy, I completely agree. The only challenge is figuring out the order of the arguments. isDivisibleBy x y sends a different message than x `isDivisibleBy` y.
    – dfeuer
    Nov 13 at 15:35






  • 1




    @dfeuer: Yeah, same problem with other names really; divides with the same definition but used as any (42 `divides`) makes just as much sense, but divides 42 is the wrong way around. This is why I prefer functions not be named such that they’re intended to be used infix, because it’s in conflict with using them in prefix; if you do want infix, an operator is preferable. (Besides, I have a personal objection to grave accents being used as quotation characters in a programming language, haha)
    – Jon Purdy
    Nov 13 at 20:27





















8














You can define function inplace:



any (n -> n `mod` 42 == 0) [1, 2, 42]





share|improve this answer





















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    2 Answers
    2






    active

    oldest

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    2 Answers
    2






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes









    6














    Composing (0==) and (`mod 42`):



    f :: [Integer] -> Bool
    
f = any ((0==).(`mod` 42))


    futher reducing parenthesis noise:



    f :: [Integer] -> Bool
    
f = any $ (0==).(`mod` 42)


    Helper function:



    f :: [Integer] -> Bool
    
f = any div42
    
  where
    
    div42 n = n `mod` 42 == 0


    Stylistically, for this function, either way seems fine. However, say you wanted to check if any values are divisible by 42 OR 52, then utilizing composition may become more obfuscated/complex/futile. Whereas using a helper function keeps things readable: div42Or52 n = n `mod` 42 == 0 || n `mod` 52 == 0. Note, I've used a where clause above, but a let in expression or a lambda are possible alternative ways to structure helper functions.



    Ultimately, its up to the developer to balance concise code with understandable code.






    share|improve this answer



















    • 2




      For point-free code, it often helps readability to factor things into many small, generic, reusable definitions. For example, in this case I might write divisibleBy d = (== 0) . (`mod` d) (“to be divisible by d means to be equal to zero modulo d”) and then use any (divisibleBy 42). Nothing wrong with using a lambda or auxiliary definition in a where clause, though.
      – Jon Purdy
      Nov 13 at 8:17






    • 1




      @JonPurdy, I completely agree. The only challenge is figuring out the order of the arguments. isDivisibleBy x y sends a different message than x `isDivisibleBy` y.
      – dfeuer
      Nov 13 at 15:35






    • 1




      @dfeuer: Yeah, same problem with other names really; divides with the same definition but used as any (42 `divides`) makes just as much sense, but divides 42 is the wrong way around. This is why I prefer functions not be named such that they’re intended to be used infix, because it’s in conflict with using them in prefix; if you do want infix, an operator is preferable. (Besides, I have a personal objection to grave accents being used as quotation characters in a programming language, haha)
      – Jon Purdy
      Nov 13 at 20:27


















    6














    Composing (0==) and (`mod 42`):



    f :: [Integer] -> Bool
    
f = any ((0==).(`mod` 42))


    futher reducing parenthesis noise:



    f :: [Integer] -> Bool
    
f = any $ (0==).(`mod` 42)


    Helper function:



    f :: [Integer] -> Bool
    
f = any div42
    
  where
    
    div42 n = n `mod` 42 == 0


    Stylistically, for this function, either way seems fine. However, say you wanted to check if any values are divisible by 42 OR 52, then utilizing composition may become more obfuscated/complex/futile. Whereas using a helper function keeps things readable: div42Or52 n = n `mod` 42 == 0 || n `mod` 52 == 0. Note, I've used a where clause above, but a let in expression or a lambda are possible alternative ways to structure helper functions.



    Ultimately, its up to the developer to balance concise code with understandable code.






    share|improve this answer



















    • 2




      For point-free code, it often helps readability to factor things into many small, generic, reusable definitions. For example, in this case I might write divisibleBy d = (== 0) . (`mod` d) (“to be divisible by d means to be equal to zero modulo d”) and then use any (divisibleBy 42). Nothing wrong with using a lambda or auxiliary definition in a where clause, though.
      – Jon Purdy
      Nov 13 at 8:17






    • 1




      @JonPurdy, I completely agree. The only challenge is figuring out the order of the arguments. isDivisibleBy x y sends a different message than x `isDivisibleBy` y.
      – dfeuer
      Nov 13 at 15:35






    • 1




      @dfeuer: Yeah, same problem with other names really; divides with the same definition but used as any (42 `divides`) makes just as much sense, but divides 42 is the wrong way around. This is why I prefer functions not be named such that they’re intended to be used infix, because it’s in conflict with using them in prefix; if you do want infix, an operator is preferable. (Besides, I have a personal objection to grave accents being used as quotation characters in a programming language, haha)
      – Jon Purdy
      Nov 13 at 20:27
















    6












    6








    6






    Composing (0==) and (`mod 42`):



    f :: [Integer] -> Bool
    
f = any ((0==).(`mod` 42))


    futher reducing parenthesis noise:



    f :: [Integer] -> Bool
    
f = any $ (0==).(`mod` 42)


    Helper function:



    f :: [Integer] -> Bool
    
f = any div42
    
  where
    
    div42 n = n `mod` 42 == 0


    Stylistically, for this function, either way seems fine. However, say you wanted to check if any values are divisible by 42 OR 52, then utilizing composition may become more obfuscated/complex/futile. Whereas using a helper function keeps things readable: div42Or52 n = n `mod` 42 == 0 || n `mod` 52 == 0. Note, I've used a where clause above, but a let in expression or a lambda are possible alternative ways to structure helper functions.



    Ultimately, its up to the developer to balance concise code with understandable code.






    share|improve this answer














    Composing (0==) and (`mod 42`):



    f :: [Integer] -> Bool
    
f = any ((0==).(`mod` 42))


    futher reducing parenthesis noise:



    f :: [Integer] -> Bool
    
f = any $ (0==).(`mod` 42)


    Helper function:



    f :: [Integer] -> Bool
    
f = any div42
    
  where
    
    div42 n = n `mod` 42 == 0


    Stylistically, for this function, either way seems fine. However, say you wanted to check if any values are divisible by 42 OR 52, then utilizing composition may become more obfuscated/complex/futile. Whereas using a helper function keeps things readable: div42Or52 n = n `mod` 42 == 0 || n `mod` 52 == 0. Note, I've used a where clause above, but a let in expression or a lambda are possible alternative ways to structure helper functions.



    Ultimately, its up to the developer to balance concise code with understandable code.







    share|improve this answer














    share|improve this answer



    share|improve this answer








    edited Nov 13 at 5:06

























    answered Nov 13 at 4:07









    DavOS

    6441621




    6441621








    • 2




      For point-free code, it often helps readability to factor things into many small, generic, reusable definitions. For example, in this case I might write divisibleBy d = (== 0) . (`mod` d) (“to be divisible by d means to be equal to zero modulo d”) and then use any (divisibleBy 42). Nothing wrong with using a lambda or auxiliary definition in a where clause, though.
      – Jon Purdy
      Nov 13 at 8:17






    • 1




      @JonPurdy, I completely agree. The only challenge is figuring out the order of the arguments. isDivisibleBy x y sends a different message than x `isDivisibleBy` y.
      – dfeuer
      Nov 13 at 15:35






    • 1




      @dfeuer: Yeah, same problem with other names really; divides with the same definition but used as any (42 `divides`) makes just as much sense, but divides 42 is the wrong way around. This is why I prefer functions not be named such that they’re intended to be used infix, because it’s in conflict with using them in prefix; if you do want infix, an operator is preferable. (Besides, I have a personal objection to grave accents being used as quotation characters in a programming language, haha)
      – Jon Purdy
      Nov 13 at 20:27
















    • 2




      For point-free code, it often helps readability to factor things into many small, generic, reusable definitions. For example, in this case I might write divisibleBy d = (== 0) . (`mod` d) (“to be divisible by d means to be equal to zero modulo d”) and then use any (divisibleBy 42). Nothing wrong with using a lambda or auxiliary definition in a where clause, though.
      – Jon Purdy
      Nov 13 at 8:17






    • 1




      @JonPurdy, I completely agree. The only challenge is figuring out the order of the arguments. isDivisibleBy x y sends a different message than x `isDivisibleBy` y.
      – dfeuer
      Nov 13 at 15:35






    • 1




      @dfeuer: Yeah, same problem with other names really; divides with the same definition but used as any (42 `divides`) makes just as much sense, but divides 42 is the wrong way around. This is why I prefer functions not be named such that they’re intended to be used infix, because it’s in conflict with using them in prefix; if you do want infix, an operator is preferable. (Besides, I have a personal objection to grave accents being used as quotation characters in a programming language, haha)
      – Jon Purdy
      Nov 13 at 20:27










    2




    2




    For point-free code, it often helps readability to factor things into many small, generic, reusable definitions. For example, in this case I might write divisibleBy d = (== 0) . (`mod` d) (“to be divisible by d means to be equal to zero modulo d”) and then use any (divisibleBy 42). Nothing wrong with using a lambda or auxiliary definition in a where clause, though.
    – Jon Purdy
    Nov 13 at 8:17




    For point-free code, it often helps readability to factor things into many small, generic, reusable definitions. For example, in this case I might write divisibleBy d = (== 0) . (`mod` d) (“to be divisible by d means to be equal to zero modulo d”) and then use any (divisibleBy 42). Nothing wrong with using a lambda or auxiliary definition in a where clause, though.
    – Jon Purdy
    Nov 13 at 8:17




    1




    1




    @JonPurdy, I completely agree. The only challenge is figuring out the order of the arguments. isDivisibleBy x y sends a different message than x `isDivisibleBy` y.
    – dfeuer
    Nov 13 at 15:35




    @JonPurdy, I completely agree. The only challenge is figuring out the order of the arguments. isDivisibleBy x y sends a different message than x `isDivisibleBy` y.
    – dfeuer
    Nov 13 at 15:35




    1




    1




    @dfeuer: Yeah, same problem with other names really; divides with the same definition but used as any (42 `divides`) makes just as much sense, but divides 42 is the wrong way around. This is why I prefer functions not be named such that they’re intended to be used infix, because it’s in conflict with using them in prefix; if you do want infix, an operator is preferable. (Besides, I have a personal objection to grave accents being used as quotation characters in a programming language, haha)
    – Jon Purdy
    Nov 13 at 20:27






    @dfeuer: Yeah, same problem with other names really; divides with the same definition but used as any (42 `divides`) makes just as much sense, but divides 42 is the wrong way around. This is why I prefer functions not be named such that they’re intended to be used infix, because it’s in conflict with using them in prefix; if you do want infix, an operator is preferable. (Besides, I have a personal objection to grave accents being used as quotation characters in a programming language, haha)
    – Jon Purdy
    Nov 13 at 20:27















    8














    You can define function inplace:



    any (n -> n `mod` 42 == 0) [1, 2, 42]





    share|improve this answer


























      8














      You can define function inplace:



      any (n -> n `mod` 42 == 0) [1, 2, 42]





      share|improve this answer
























        8












        8








        8






        You can define function inplace:



        any (n -> n `mod` 42 == 0) [1, 2, 42]





        share|improve this answer












        You can define function inplace:



        any (n -> n `mod` 42 == 0) [1, 2, 42]






        share|improve this answer












        share|improve this answer



        share|improve this answer










        answered Nov 13 at 4:12









        talex

        9,1031546




        9,1031546






























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