hashmap - Nested Map in Haskell -


what trying when have

rec1 = [("name", "obj-1"), ("status", "up"), ("region", "us")]

then have

[( "us", [("up", [("name", "obj-1"), ("status", "up"), ("region", "us")])])]

this code have come in haskell.

rec1 = [("name", "obj-1"), ("status", "up"), ("region", "us")]  convertto rec = prelude.foldl (\map (k, v) -> data.map.insert k v map) data.map.empty rec  justfold = prelude.foldr (\key val -> data.map.insert key val data.map.empty) (convertto rec1) ["us","up"]

in third line above, in lambda function definition, compiler not agreeing type of val. saying expects char , getting string or not able match map string list type.

i not haskell, looking if give me pointers it.

thanks.

edit: corrected output type. mistyped @ first.

there few issues code, easy, , hard. let's examine easy ones first:

the first convertto function. exists already, , called fromlist. there's function creating new map 1 pair in it, called singleton. way you're referring functions in data.map unusual. fixing these issues gives:

import qualified data.map m  rec1 = m.fromlist [("name", "obj-1"), ("status", "up"), ("region", "us")]  justfold = foldr m.singleton rec1 ["us","up"]

now however, there's deeper problem. figure out is, let's try adding type signatures rec1 , justfold:

import qualified data.map m  rec1 :: m.map [char] [char] rec1 = m.fromlist [("name", "obj-1"), ("status", "up"), ("region", "us")]   justfold :: m.map [char] (m.map [char] (m.map [char] [char])) justfold = foldr m.singleton rec1 ["us","up"]

rec1 fine, justfold problem. see why, @ type signatures of foldr , singleton:

foldr :: (a -> b -> b) -> b -> [a] -> b  singleton :: k -> -> map k

foldr requires function of type (a -> b -> b), , singleton not that, because a , map k a not same thing. following code produces result want:

import qualified data.map m  rec1 = m.fromlist [("name", "obj-1"), ("status", "up"), ("region", "us")]  justfold = m.singleton "us" $ m.singleton "up" rec1

however, fundamental issue can't iterate on changes type on every iteration. if find wanting this, advise take step , rethink approach problem.


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