Haskell: Multiple declaration of datatype -


this question has answer here:

i have little problem while creating data types.

i have 2 data types, football , tennis

data football  = football   { players       :: players       , stadium       :: stadium    }       data tennis  = tennis   { players1      :: players       , stadium1      :: stadium    }

and want create other datatype sport using constructor datatype football , tennis data sport = football | tennis a , following error, multiple declarations of football don't understand doing wrong.

best regards

your idea correct in spirit haskell uses tagged unions (also called "discriminated unions", "disjoint unions", "variants", , "sum types"). means need additional tag determine if sport constructed football or if constructed tennis.

data sport = sportfootball football | sporttennis tennis

these tags known in haskell data constructors. is, defines constructor sportfootball :: football -> sport , constructor sporttennis :: tennis -> sport.

suppose haskell did not require tag, type?

data untaggedint = int | int

an untagged union should have property (x | x) = x , must equivalent to:

data untaggedint = int

whereas tagged unions can define:

data taggedint = int | int

that is, tagged unions not idempotent.

also consider else happens without constructors. suppose have definition such as:

data untaggedlists = string | [string]

and tasked finding type of expression "hello world". should type string or untaggedlists? suppose have similar definition:

data anotheruntaggedlists = string | [string]

is true untaggedlists , anotheruntaggedlists equal types?

these not unanswerable questions, demonstrate profound systematic difference between having tags , not.


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