CODEWITHSUNDEEP
haskell
user7361363
user7361363

Reputation:

Haskell, modyfying only one field of structure

I have a some structure: 

data S = { 
  a :: Integer,
  b :: Integer,
  c :: String,
  d :: Map String Integer
}

I closed it in my State monad, And I am using it in following way: 

s <- get
let S {a = a', b =b', c=c', d=d'} s   
put $ S {a = a' + 1, b = b', c= c', d = d'}

As you can see, when I would like to modify/read only one field (here a), I must get and put all fields - it is embarassing - otherwise I get warnings and sometimes runtime errors.

Can you try to propose me some solution to make it better and cleaner ?

Thanks in advance,
Regards

Upvotes: 3

Views: 77

Answers (1)

Alec
Alec

Reputation: 32319

You can use a record update and specify only the fields you want to update. Also, you can extract a particular field during pattern matching.

s@S{ a = a' } <- get 
put $ s{ a = a' + 1 }

Alternately, if you find yourself doing a lot of this sort of thing, you might want to use lens. With lens, you would have to make lenses for S (which requires turning on -XTemplateHaskell)

data S = { _a, _b :: Integer, _c :: String, _d :: Map String Integer }
makeLenses ''S

But then, in your state monad you can just write

a += 1

Yep, this is legitimate Haskell. += is actually a thing.

Upvotes: 7

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