Generalised plusequals

5 comments

• flebron 2 hours ago
  The website asks what they do in Haskell. The answer is property modification and reading, as well as very powerful traversal constructs, use lenses (https://hackage.haskell.org/package/lens , tutorial at https://hackage.haskell.org/package/lens-tutorial-1.0.5/docs...).
• RodgerTheGreat 1 hour ago
  In Lil[0], this is how ordinary assignment syntax works. Implicitly defining a dictionary stored in a variable named "cat" with a field "age":

      cat.age:3
      # {"age":3}
  

  Defining "l" as in the example in the article. We need the "list" operator to enlist nested values so that the "," operator doesn't concatenate them into a flat list:

      l:1,(list 2,list cat),4
      # (1,(2,{"age":3}),4)
  

  Updating the "age" field in the nested dictionary. Lil's basic datatypes are immutable, so "l" is rebound to a new list containing a new dictionary, leaving any previous references undisturbed:

      l[1][1].age:9
      # (1,(2,{"age":9}),4)
      cat
      # {"age":3}
  

  There's no special "infix" promotion syntax, so that last example would be:

      l:l,5
      # (1,(2,{"age":9}),4,5)
  

  [0] http://beyondloom.com/tools/trylil.html
• hatthew 2 hours ago
  It seems like this is proposing syntactic sugar to make mutating and non-mutating operations be on equal footing.

  > The more interesting example is reassigning the deeply nested l to make the cat inside older, without mutating the original cat

  Isn't that mutating l, though? If you're concerned about mutating cat, shouldn't you be concerned about mutating l?

  [-]
  • two_handfuls 2 hours ago
    It doesn't mutate l exactly, it makes a new list slightly different from the original one and assigns it to l.

    That means if someone has a reference to the original l, they do not see the change (because l is immutable. Both of them).
• rokob 2 hours ago
  Yeah this looks like lenses at first glance
• beaumayns 2 hours ago
  q has the concept of amend, which is similar: https://code.kx.com/q4m3/6_Functions/#683-general-form-of-fu...

  It's quite handy, though the syntax for it is rather clunky compared to the rest of the language in my opinion.