How to access fields of a Union in Elm? -

i learning programming language elm. want access particular field of union. unable access it. have searched in documentation. not find anywhere how access particular field of union. code follows:

import graphics.element exposing (show) import list exposing (length, filter, map)  type person    = person { fname: string, lname: string, age: int}  p1 = person { fname="abc", lname="xyz", age=23 } p2 = person { fname="jk", lname="rowling", age=24 }  -- these unions fields  people : list person people = [ p1          , p2          , person {fname= "anakin", lname= "luke", age= 12}          ]  main = show [people] 

i cannot use p1.name since p1 not record. how access field fname of p1 or p2?

the documentation on website still getting improved, , need more links between different parts. documentation you're looking here.

case expressions

the relevant part explains union type can have different tagged values, need case-expression distinguish different options:

case p1 of   person r -> r.fname 

let expressions

since in case there 1 option, can safely use deconstruction pattern in variable assignment:

(person p1record) = p1 -- p1record defined, , can use p1record.fname 

be careful let expressions

note if use assignment trick union type multiple tags, open program runtime crashes. crash happen if value try deconstruct not of right tag:

type direction   = left   | right   | forward   |  march : direction -> boolean march dir =   let     forward = dir -- contrived example   in     true  main =   show (march back) -- oops, runtime crash 

final note

since union type has 1 tag, may want consider using type alias instead. it's utility write nicer name type, don't have unwrap use inner type:

type alias person    = {fname: string, lname: string, age: int}  p1 = {fname= "abc", lname= "xyz", age= 23} -- p1.fname accessible