I've been trying to compare two lists in Haskell and found an answer here.
I wonder how all (flip elem listx) input works, especially for the role flip plays here.
When I take out flip it won't work anymore.
1 Answer
flip elem listxis equivalent to(flip elem) listx.(flip elem)is the same aselem, but with the arguments in opposite order. This is whatflipdoes.elemis a function that takes an element and a list, and checks whether the element belongs to the list.- So
flip elemis a function that that takes a list and an element, and checks whether the element belongs to the list. - Therefore
flip elem listxis a function that that takes an element, and checks whether the element belongs tolistx. - Now
alltakes a predicate and a list, and checks whether all elements of the list satisfy the predicate. all (flip elem listx)take a list, and checks whether all elements of the list satisfyflip elem listx. That is, whether they all belong tolistx.all (flip elem listx) inputchecks whether all elements ofinputbelong tolistx.- Q.E.D.
answered Oct 1, 2011 at 9:06
n. m. could be an AI
122k14 gold badges141 silver badges267 bronze badges
all (`elem` listx) input. If you know about backticks and operator sections this should make sense.e ´elem´ listjust reads better thanelem e listmany prefer it this way.inputisn't an argument toelem, quite. Read left to right.alltakes two arguments, a function and a list, and returns true if the function is true for everything in the list. The arguments to elem are listx and one element at a time from input.