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.


Comments

Post a Comment

Popular posts from this blog

python - Operations inside variables -

this question has answer here: evaluating mathematical expression in string 12 answers i want make python script finds maximum possible output using 2 operations. problem says there syntax error @ 2nd last line, in b in there. how can fix it? x = 0 = int(input("1st number:")) c = int(input("2nd number:" )) e = int(input("3rd number:" )) in range(4): if == 0: b = "+" elif == 1: b = "-" elif == 2: b = "/" else: b = "*" j in range(4): if j == 0: d = "+" elif j == 1: d = "-" elif j == 2: d = "/" else: d = "*" k = b c d e print(k) if you're after variables e on single line, couldn...
Read more

Generic Map Parameter java -

i having lot of methods of type: public static list<eduusers> getdetails(class c, map<string, integer> params) { list<eduusers> details = null; session session = hibernateutil.getsessionfactory().opensession(); transaction tx = null; try { tx = session.begintransaction(); criteria cr = session.createcriteria(c); (map.entry<string, integer> entry : params.entryset()) { cr.add(restrictions.eq(entry.getkey(), entry.getvalue())); } details = cr.list(); tx.commit(); } catch (exception asd) { log.debug(asd.getmessage()); if (tx != null) { tx.commit(); } } { session.close(); } return details; } i trying have generic method them , have written far: public static <t> list<t> getdetails(class<t> c, class<t> b) { l...
Read more

arrays - What causes a java.lang.ArrayIndexOutOfBoundsException and how do I prevent it? -

what arrayindexoutofboundsexception mean , how rid of it? here code sample triggers exception: string[] name = {"tom", "dick", "harry"}; for(int = 0; i<=name.length; i++) { system.out.print(name[i] +'\n'); } your first port of call should documentation explains reasonably clearly: thrown indicate array has been accessed illegal index. index either negative or greater or equal size of array. so example: int[] array = new int[5]; int boom = array[10]; // throws exception as how avoid it... um, don't that. careful array indexes. one problem people run thinking arrays 1-indexed, e.g. int[] array = new int[5]; // ... populate array here ... (int index = 1; index <= array.length; index++) { system.out.println(array[index]); } that miss out first element (index 0) , throw exception when index 5. valid indexes here 0-4 inclusive. correct, idiomatic for statement here be: for (int index = 0; index <...
Read more