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What is the type signature for json hashmap in Haskell and how using it is different from using a dynamically typed language?

chowells 9y ago
The easiest way to use JSON as dynamically typed data in Haskell is with the lens[1] and lens-aeson[2] packages. The type signatures those use are astounding, and not trivial to understand. But the code that results is very simple while remaining strongly typed. It ends up being not that different in style from something like hpricot in ruby. [1] https://hackage.haskell.org/package/lens https://hackage.haskell.org/package/lens [2] https://hackage.haskell.org/package/lens-aeson https://hackage.haskell.org/package/lens-aeson
platz 9y ago
A json object has the type signature: type Object = HashMap Text Value which is A JSON "object" (key/value map). The keys of the hashmap are Text and the leaves are "Value" which contain more JSON here are different well-formed json values that both resolve to the same type, A.Object (either you get a parse error String for not-well-formed json, or an A.Object) >>> A.eitherDecode "{ \"name\": \"Joe\", \"age\": 12 }" :: Either String A.Object Right (fromList [("age",Number 12.0),("name",String "Joe")]) >>> A.eitherDecode "{ \"name\": \"Joe\", \"age\": {\"foo\": \"12\"} }" :: Either String A.Object Right (fromList [("age",Object (fromList [("foo",String "12")])),("name",String "Joe")]) Then you can access values in the HashMap by key just like a hashmap in any other language. Or, as noted by chowells, you can use lens-aeson which can be thought of as a DSL to perform queries in a dynamic fashion: retrieve value of "foo" in "age" >>> s = "{ \"name\": \"Joe\", \"age\": {\"foo\": \"12\"} }" :: String >>> s ^? key "age" . key "foo" Just (String "12")
tome 9y ago
> how using it is different from using a dynamically typed language It's not different for that type. The point of using a statically typed language is you can give static types to other entities in your program!

chowells 9y ago

The easiest way to use JSON as dynamically typed data in Haskell is with the lens[1] and lens-aeson[2] packages. The type signatures those use are astounding, and not trivial to understand. But the code that results is very simple while remaining strongly typed. It ends up being not that different in style from something like hpricot in ruby. [1] https://hackage.haskell.org/package/lens https://hackage.haskell.org/package/lens [2] https://hackage.haskell.org/package/lens-aeson https://hackage.haskell.org/package/lens-aeson

platz 9y ago

A json object has the type signature: type Object = HashMap Text Value which is A JSON "object" (key/value map). The keys of the hashmap are Text and the leaves are "Value" which contain more JSON here are different well-formed json values that both resolve to the same type, A.Object (either you get a parse error String for not-well-formed json, or an A.Object) >>> A.eitherDecode "{ \"name\": \"Joe\", \"age\": 12 }" :: Either String A.Object Right (fromList [("age",Number 12.0),("name",String "Joe")]) >>> A.eitherDecode "{ \"name\": \"Joe\", \"age\": {\"foo\": \"12\"} }" :: Either String A.Object Right (fromList [("age",Object (fromList [("foo",String "12")])),("name",String "Joe")]) Then you can access values in the HashMap by key just like a hashmap in any other language. Or, as noted by chowells, you can use lens-aeson which can be thought of as a DSL to perform queries in a dynamic fashion: retrieve value of "foo" in "age" >>> s = "{ \"name\": \"Joe\", \"age\": {\"foo\": \"12\"} }" :: String >>> s ^? key "age" . key "foo" Just (String "12")

tome 9y ago

> how using it is different from using a dynamically typed language It's not different for that type. The point of using a statically typed language is you can give static types to other entities in your program!