list - Alternating Sum Using FOLDR (SML) -

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I use foldr to get an optional amount from the list I'm trying to int, but keep an operator / operator mismatch error. I think I'm expecting the output to be fold / code> 

  1 - 3 + 5 - 7 + 9 = 5

Edit:

I actually asked to use an anonymous function with foldr

For example, for example 

  l = [1,3,5,7,9] fold (FN (x, y): => x - y) l

with the same expected output, but I'm still merging the same operator / operator

foldr code> foldl ) Once Rnbhik value.

foldr type ('a *' b -> 'b') - gt; 'B - & gt; 'A list - & gt; 'B' , where 'b is the result of the result of the bottom of each stage (also the initial value, which is the initial "result"); And the 'a is the type of elements in the list. Note that the combination function can be different from 'a and ' b takes an element and previous result and gives new results.

However, you need to think about a suitable initial value. What should this be? I think 0 would do:

fold-op-0 l

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