Better explanation of flip function #260

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1. milesfrain commented on Dec 21, 2020

   

   milesfrain

   on Dec 21, 2020

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   Excellent feedback. Thanks.

   I recall flip being pretty mind-melting for me when learning Elm the first time, but having paid those FP dues, I didn't catch this as being confusing in the PureScript book.

   I think the solution is to fix the library docs and perhaps link to the Pursuit entry from the book. I'm not a fan of the existing Pursuit example, since const is pretty weird too. Opened purescript/purescript-prelude#237, which is up for grabs. Let me know if you want to tackle it, otherwise I can make the PR.

   Also, great workaround for linking to lines in markdown. Annoying that isaacs/github#297 has been on the wish list for the past 6 years.
2. milesfrain commented on Jan 2, 2021

   

   milesfrain

   on Jan 2, 2021

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   I think flip would make more sense to cover in Ch4 when discussing map.
   It's also a good compliment to operator sections, which is not covered yet in the book. Not sure if the book should cover this syntax in detail, or if we can just link to the language reference.

   Something like this could be a good sequence:

   ---

   Let's say we want to perform truncating division by 5 on each element of an array of integers.

   > arr = range 0 20
   > arr
   [0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20]
   

   This first attempt is not quite right:

   > map (div 5) arr
   [0,5,2,1,1,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0]
   

   We're mixing up the argument order and dividing 5 by each element of the array instead.
   We can fix that by indicating that each array element needs to be the first argument to div:

   > map (\n -> div n 5) arr
   [0,0,0,0,0,1,1,1,1,1,2,2,2,2,2,3,3,3,3,3,4]
   

   It's more concise with div's infix operator /:

   > map (\n -> n / 5) arr
   [0,0,0,0,0,1,1,1,1,1,2,2,2,2,2,3,3,3,3,3,4]
   

   But most concise by using _ as an operator section:

   > map (_ / 5) arr
   [0,0,0,0,0,1,1,1,1,1,2,2,2,2,2,3,3,3,3,3,4]
   

   flip is another way to swap the ordering of a function's arguments:

   > map (flip div 5) arr
   [0,0,0,0,0,1,1,1,1,1,2,2,2,2,2,3,3,3,3,3,4]
   
4. JordanMartinez commented on Apr 28, 2021

   

   JordanMartinez

   on Apr 28, 2021

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   Would this orientation help?

   flip :: forall firstArg secondArg returnType
      . (firstArg -> secondArg -> returnType)
     -> secondArg
     -> firstArg
     -> returnType
   flip functionABToC b a = functionABToC a b  
   
   append "hello" "world" == "helloworld"
   
   flip append "hello" "world" == "worldhello"
   
   -- flip's type signature can be simplified
   flip :: forall a b c.
      . (a -> b -> c)
     -> b
     -> a
     -> c
   flip f b a = f a b  
6. Zelenya

   closed this as completedin #446on Jul 17, 2023