learn you some erlang - chap 6 to chap7

learn you some erlang - chap 6 to chap7

• 1.
  LEARN U ERLANG WEEK3- PRESENTED BY NOLLEH
• 2.
  6. HIGHER ORDERFUNCTIONS LET’S GET FUNCTIONAL ▸ in functional programming language… ▸ ability to take function ▸ used like variable within function ▸ THIS IS HIGHER ORDER FUNCTION! ▸ lambda calculus ▸ it is originated from mathematics ▸ system, see everything as function, even numbers
• 3.
  ▸ higher orderfunction - add/2 6. HIGHER ORDER FUNCTIONS LET’S GET FUNCTIONAL -module(hhfuns). -compile(export_all). one() -> 1. two() -> 2. add(X,Y) -> X() + Y().
• 4.
  ▸ higher orderfunction - add/2 6. HIGHER ORDER FUNCTIONS LET’S GET FUNCTIONAL 4> hhfuns:add(fun hhfuns:one/0, fun hhfuns:two/0). 3 fun Module:Function/Arity
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  6. HIGHER ORDERFUNCTIONS LET’S GET FUNCTIONAL ▸ where comes from it’s strength? ▸ we will change these functions a bit generally. increment([]) -> []; increment([H|T]) -> [H+1|increment(T)]. decrement([]) -> []; decrement([H|T]) -> [H-1|decrement(T)].
• 6.
  6. HIGHER ORDERFUNCTIONS LET’S GET FUNCTIONAL ▸ look at that! ▸ very smart abstraction! map(_, []) -> []; map(F, [H|T]) -> [F(H)|map(F,T)]. incr(X) -> X + 1. decr(X) -> X - 1. 5> hhfuns:map(fun hhfuns:incr/1, [1,2,3,4,5]). [2,3,4,5,6] 6> hhfuns:map(fun hhfuns:decr/1, [1,2,3,4,5]). [0,1,2,3,4]
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  6. HIGHER ORDERFUNCTIONS ANONYMOUS FUNCTIONS ▸ function inline fun(Args1) -> Expression1, Exp2, ..., ExpN; (Args2) -> Expression1, Exp2, ..., ExpN; (Args3) -> Expression1, Exp2, ..., ExpN end
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  6. HIGHER ORDERFUNCTIONS ANONYMOUS FUNCTIONS ▸ map with lambda ▸ the loop logic is can be ignored. instead, focused on what will be done to elements 9> hhfuns:map(fun(X) -> X + 1 end, L). [2,3,4,5,6] 10> hhfuns:map(fun(X) -> X - 1 end, L). [0,1,2,3,4]
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  6. HIGHER ORDERFUNCTIONS ANONYMOUS FUNCTIONS ▸ closure 11> PrepareAlarm = fun(Room) -> 11> io:format("Alarm set in ~s.~n",[Room]), 11> fun() -> io:format("Alarm tripped in ~s! Call Batman! ~n",[Room]) end 11> end. #Fun<erl_eval.20.67289768> 12> AlarmReady = PrepareAlarm("bathroom"). Alarm set in bathroom. #Fun<erl_eval.6.13229925> 13> AlarmReady(). Alarm tripped in bathroom! Call Batman! ok
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  6. HIGHER ORDERFUNCTIONS ANONYMOUS FUNCTIONS ▸ understand scope ▸ F inherits base/1's scope base(A) -> B = A + 1, F = fun() -> A * B end, F().
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  6. HIGHER ORDERFUNCTIONS ANONYMOUS FUNCTIONS ▸ a parent cannot inherits it’s decent’s scope base(A) -> B = A + 1, F = fun() -> C = A * B end, F(), C.
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  6. HIGHER ORDERFUNCTIONS ANONYMOUS FUNCTIONS ▸ carry it’s context a() -> Secret = "pony", fun() -> Secret end. b(F) -> "a/0's password is "++F(). 14> c(hhfuns). {ok, hhfuns} 15> hhfuns:b(hhfuns:a()). "a/0's password is pony"
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  6. HIGHER ORDERFUNCTIONS ANONYMOUS FUNCTIONS ▸ shadowing base() -> A = 1, (fun() -> A = 2 end)(). base() -> A = 1, (fun(A) -> A = 2 end)(2).
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  6. HIGHER ORDERFUNCTIONS MAPS, FILTERS, FOLDS AND MORE ▸ like map, you can abstract functions like even, old_man.. even(L) -> lists:reverse(even(L,[])). even([], Acc) -> Acc; even([H|T], Acc) when H rem 2 == 0 -> even(T, [H|Acc]); even([_|T], Acc) -> even(T, Acc). old_men(L) -> lists:reverse(old_men(L,[])). old_men([], Acc) -> Acc; old_men([Person = {male, Age}|People], Acc) when Age > 60 -> old_men(People, [Person|Acc]); old_men([_|People], Acc) -> old_men(People, Acc).
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  6. HIGHER ORDERFUNCTIONS MAPS, FILTERS, FOLDS AND MORE ▸ extract the common parts ▸ try to get rid of what's always the same and let the programmer supply in the parts that change. filter(Pred, L) -> lists:reverse(filter(Pred, L,[])). filter(_, [], Acc) -> Acc; filter(Pred, [H|T], Acc) -> case Pred(H) of true -> filter(Pred, T, [H|Acc]); false -> filter(Pred, T, Acc) end.
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  6. HIGHER ORDERFUNCTIONS MAPS, FILTERS, FOLDS AND MORE ▸ use it! 1> c(hhfuns). 2> Numbers = lists:seq(1,10). 3> hhfuns:filter(fun(X) -> X rem 2 == 0 end, Numbers). [2,4,6,8,10] 4> People = [{male,45},{female,67},{male,66},{female,12},{unknown, 174},{male,74}]. [{male,45},{female,67},{male,66},{female,12},{unknown,174},{male,74}] 5> hhfuns:filter(fun({Gender,Age}) -> Gender == male andalso Age > 60 end, People). [{male,66},{male,74}]
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  6. HIGHER ORDERFUNCTIONS MAPS, FILTERS, FOLDS AND MORE ▸ fold - it reduces to one value %% find the maximum of a list max([H|T]) -> max2(T, H). max2([], Max) -> Max; max2([H|T], Max) when H > Max -> max2(T, H); max2([_|T], Max) -> max2(T, Max).
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  6. HIGHER ORDERFUNCTIONS MAPS, FILTERS, FOLDS AND MORE ▸ fold - it reduces to one value %% find the minimum of a list min([H|T]) -> min2(T,H). min2([], Min) -> Min; min2([H|T], Min) when H < Min -> min2(T,H); min2([_|T], Min) -> min2(T, Min).
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  6. HIGHER ORDERFUNCTIONS MAPS, FILTERS, FOLDS AND MORE ▸ fold - it reduces to one value %% sum of all the elements of a list sum(L) -> sum(L,0). sum([], Sum) -> Sum; sum([H|T], Sum) -> sum(T, H+Sum).
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  6. HIGHER ORDERFUNCTIONS MAPS, FILTERS, FOLDS AND MORE max([H|T]) -> max2(T, H). max2([], Max) -> Max; max2([H|T], Max) when H > Max -> max2(T, H); max2([_|T], Max) -> max2(T, Max). min([H|T]) -> min2(T,H). min2([], Min) -> Min; min2([H|T], Min) when H < Min -> min2(T,H); min2([_|T], Min) -> min2(T, Min). sum(L) -> sum(L,0). sum([], Sum) -> Sum; sum([H|T], Sum) -> sum(T, H+Sum).
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  6. HIGHER ORDERFUNCTIONS MAPS, FILTERS, FOLDS AND MORE ▸ fold - it reduces to one value fold(_, Start, []) -> Start; fold(F, Start, [H|T]) -> fold(F, F(H,Start), T).
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  6. HIGHER ORDERFUNCTIONS MAPS, FILTERS, FOLDS AND MORE ▸ use it! 6> c(hhfuns). {ok, hhfuns} 7> [H|T] = [1,7,3,5,9,0,2,3]. [1,7,3,5,9,0,2,3] 8> hhfuns:fold(fun(A,B) when A > B -> A; (_,B) -> B end, H, T). 9 9> hhfuns:fold(fun(A,B) when A < B -> A; (_,B) -> B end, H, T). 0 10> hhfuns:fold(fun(A,B) -> A + B end, 0, lists:seq(1,6)). 21
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  6. HIGHER ORDERFUNCTIONS MAPS, FILTERS, FOLDS AND MORE reverse(L) -> fold(fun(X,Acc) -> [X|Acc] end, [], L). map2(F,L) -> reverse(fold(fun(X,Acc) -> [F(X)|Acc] end, [], L)). filter2(Pred, L) -> F = fun(X,Acc) -> case Pred(X) of true -> [X|Acc]; false -> Acc end end, reverse(fold(F, [], L)).
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  6. HIGHER ORDERFUNCTIONS MAPS, FILTERS, FOLDS AND MORE ▸ U can using it from Standard library list:map/2, list:filter/2, list:foldl/3, list:foldr/3 all/2, any/2 dropwhile/2, takewhile/2, partition/2, flatten/1, flatlength/1, flatmap/2, merge/1, nth/2, nthtail/2, split/2
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  7. ERRORS ANDEXCEPTIONS NOT SO FAST! ▸ you are probably running into errors ▸ can’t explain all error handling, now ▸ erlang has 2 paradigm ▸ functional / concurrent ▸ even thought, keep it mind that… let it crash~
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  7. ERRORS ANDEXCEPTIONS A COMPILATION OF ERRORS ▸ syntactic mistake ▸ module.beam:module name does not match file name ‘module’ ▸ ./module.erl:2: Warning: function some_function/0 is unused ▸ not export, used with wrong name.. ▸ ./module.erl:2: function some_function/1 undefined ▸ wrong name, arity, function could not be compiled..(forgot period)
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  7. ERRORS ANDEXCEPTIONS A COMPILATION OF ERRORS ▸ ./module.erl:5: syntax error before: ‘SomeCharacterOrWord' ▸ unclosed parenthesis, wrong expression termination, encoding… ▸ ./module.erl:5: syntax error before: ▸ line termination is not correct (specific case of previous error) ▸ ./module.erl:5: Warning: this expression will fail with a 'badarith' exception ▸ llama + 5 ▸ ./module.erl:5: Warning: variable 'Var' is unused
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  7. ERRORS ANDEXCEPTIONS A COMPILATION OF ERRORS ▸ ./module.erl:5: head mismatch ▸ function has more than one head, which is different, i.e. arty ▸ ./module.erl:5: Warning: this clause cannot match because a previous clause at line 4 always matches ▸ ./module.erl:9: variable 'A' unsafe in 'case' (line 5) ▸ using a variable declared within branch of case … of ▸ if you want to use, MyVar = case ... of...
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  7. ERRORS ANDEXCEPTIONS NO! YOUR LOGIC IS WRONG! ▸ using debug tool ▸ test framework, tracing module… ▸ It's easier to focus on those that make your programs crash ▸ won't bubble up 50 levels from now
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  7. ERRORS ANDEXCEPTIONS RUNTIME ERRORS ▸ crash your code ▸ function_clause ▸ case_clause 1> lists:sort([3,2,1]). [1,2,3] 2> lists:sort(fffffff). ** exception error: no function clause matching lists:sort(fffffff) 3> case "Unexpected Value" of 3> expected_value -> ok; 3> other_expected_value -> 'also ok' 3> end.
• 31.
  7. ERRORS ANDEXCEPTIONS RUNTIME ERRORS ▸ if_clause ▸ badmatch 4> if 2 > 4 -> ok; 4> 0 > 1 -> ok 4> end. ** exception error: no true branch found when evaluating an if expression 5> [X,Y] = {4,5}. ** exception error: no match of right hand side value {4,5}
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  7. ERRORS ANDEXCEPTIONS RUNTIME ERRORS ▸ badarg ▸ undef 6> erlang:binary_to_list("heh, already a list"). ** exception error: bad argument in function binary_to_list/1 called as binary_to_list("heh, already a list") 7> lists:random([1,2,3]). ** exception error: undefined function lists:random/1
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  7. ERRORS ANDEXCEPTIONS RUNTIME ERRORS ▸ badarith ▸ badfun 8> 5 + llama. ** exception error: bad argument in an arithmetic expression in operator +/2 called as 5 + llama 9> hhfuns:add(one,two). ** exception error: bad function one in function hhfuns:add/2
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  7. ERRORS ANDEXCEPTIONS RUNTIME ERRORS ▸ badarity ▸ system_limit ▸ atom too large, too many arguments, # of atom is too large… ▸ http://www.erlang.org/doc/efficiency_guide/ advanced.html#id2265856 10> F = fun(_) -> ok end. #Fun<erl_eval.6.13229925> 11> F(a,b). ** exception error: interpreted function with arity 1 called with two arguments
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  7. ERRORS ANDEXCEPTIONS RAISING EXCEPTIONS ▸ let it crash! ▸ 3 kind of exceptions ▸ errors ▸ exits ▸ throws
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  7. ERRORS ANDEXCEPTIONS RAISING EXCEPTIONS ▸ errors ▸ end execution of current process ▸ include stacktrace ▸ the error what you’ve seen in previous clause. ▸ do not use where user must take care (tree lookup) ▸ {ok, value} / undefined… erlang:error(Reason)
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  7. ERRORS ANDEXCEPTIONS RAISING EXCEPTIONS ▸ define you’re own kind of error ▸ can be handled in same manner 1> erlang:error(badarith). ** exception error: bad argument in an arithmetic expression 2> erlang:error(custom_error). ** exception error: custom_error
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  7. ERRORS ANDEXCEPTIONS RAISING EXCEPTIONS ▸ exits ▸ internal (exit/1) / external (exit/2) ▸ roughly, has same use case with error ▸ what to use ?
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  7. ERRORS ANDEXCEPTIONS RAISING EXCEPTIONS ▸ understand concept of actors and processes ▸ listen, pass msg
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  7. ERRORS ANDEXCEPTIONS RAISING EXCEPTIONS ▸ understand concept of actors and processes ▸ communicate exceptions ▸ the real deference is intent (simply error or terminate?) ▸ errors - contain stack trace / exit - not contain stack trace
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  7. ERRORS ANDEXCEPTIONS RAISING EXCEPTIONS ▸ throws ▸ programmer can be expected to handle ▸ recommend documenting them ▸ used for non-local returns in deep recursion 1> throw(permission_denied). ** exception throw: permission_denied
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  7. ERRORS ANDEXCEPTIONS DEALING WITH EXCEPTIONS ▸ try…catch try Expression of SuccessfulPattern1 [Guards] -> Expression1; SuccessfulPattern2 [Guards] -> Expression2 catch TypeOfError:ExceptionPattern1 -> Expression3; TypeOfError:ExceptionPattern2 -> Expression4 end.
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  7. ERRORS ANDEXCEPTIONS DEALING WITH EXCEPTIONS ▸ protected ▸ any exception happening within the call will be caught ▸ replace TypeOfError error, exit, throw.. ▸ default is throw
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  7. ERRORS ANDEXCEPTIONS DEALING WITH EXCEPTIONS sword(1) -> throw(slice); sword(2) -> erlang:error(cut_arm); sword(3) -> exit(cut_leg); sword(4) -> throw(punch); sword(5) -> exit(cross_bridge). black_knight(Attack) when is_function(Attack, 0) -> try Attack() of _ -> "None shall pass." catch throw:slice -> "It is but a scratch."; error:cut_arm -> "I've had worse."; exit:cut_leg -> "Come on you pansy!"; _:_ -> "Just a flesh wound." end.
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  7. ERRORS ANDEXCEPTIONS DEALING WITH EXCEPTIONS ▸ example 9> exceptions:black_knight(fun exceptions:talk/0). "None shall pass." 10> exceptions:black_knight(fun() -> exceptions:sword(1) end). "It is but a scratch." 11> exceptions:black_knight(fun() -> exceptions:sword(2) end). "I've had worse." 12> exceptions:black_knight(fun() -> exceptions:sword(3) end). "Come on you pansy!" 13> exceptions:black_knight(fun() -> exceptions:sword(4) end). "Just a flesh wound." 14> exceptions:black_knight(fun() -> exceptions:sword(5) end). "Just a flesh wound."
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  7. ERRORS ANDEXCEPTIONS DEALING WITH EXCEPTIONS ▸ _:_ patttern catch all ▸ finally - after ▸ cannot get return value from this try Expr of Pattern -> Expr1 catch Type:Exception -> Expr2 after % this always gets executed Expr3 end
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  7. ERRORS ANDEXCEPTIONS DEALING WITH EXCEPTIONS ▸ multiple protection whoa() -> try talk(), _Knight = "None shall Pass!", _Doubles = [N*2 || N <- lists:seq(1,100)], throw(up), _WillReturnThis = tequila of tequila -> "hey this worked!" catch Exception:Reason -> {caught, Exception, Reason} end.
• 48.
  7. ERRORS ANDEXCEPTIONS DEALING WITH EXCEPTIONS ▸ when return value is not useful, remove of part im_impressed() -> try talk(), _Knight = "None shall Pass!", _Doubles = [N*2 || N <- lists:seq(1,100)], throw(up), _WillReturnThis = tequila catch Exception:Reason -> {caught, Exception, Reason} end.
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  7. ERRORS ANDEXCEPTIONS DEALING WITH EXCEPTIONS ▸ protected part cannot be tail recursive ▸ of - catch space can be tail recursive
• 50.
  7. ERRORS ANDEXCEPTIONS WAIT, THERE’S MORE! ▸ keyword catch ▸ basically captures all types of exceptions on top of the good results. 1> catch throw(whoa). whoa 2> catch exit(die). {'EXIT',die}
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  7. ERRORS ANDEXCEPTIONS WAIT, THERE’S MORE! ▸ both ‘exit’ - backward compatibility 3> catch 1/0. {'EXIT',{badarith,[{erlang,'/',[1,0]}, {erl_eval,do_apply,5}, {erl_eval,expr,5}, {shell,exprs,6}, {shell,eval_exprs,6}, {shell,eval_loop,3}]}} 4> catch 2+2. 4
• 52.
  7. ERRORS ANDEXCEPTIONS WAIT, THERE’S MORE! ▸ undef:type of error ▸ stack trace {Module, Function, Arguments} ▸ stack trace {Module, Function, Arity}. ▸ erlang:get_stacktrace/0 5> catch doesnt:exist(a,4). {'EXIT',{undef,[{doesnt,exist,[a,4]}, {erl_eval,do_apply,5}, {erl_eval,expr,5}, {shell,exprs,6}, {shell,eval_exprs,6}, {shell,eval_loop,3}]}}
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  7. ERRORS ANDEXCEPTIONS WAIT, THERE’S MORE! ▸ common manner using catch catcher(X,Y) -> case catch X/Y of {'EXIT', {badarith,_}} -> "uh oh"; N -> N end.
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  7. ERRORS ANDEXCEPTIONS WAIT, THERE’S MORE! ▸ common manner using catch 7> exceptions:catcher(3,3). 1.0 8> exceptions:catcher(6,3). 2.0 9> exceptions:catcher(6,0). "uh oh"
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  7. ERRORS ANDEXCEPTIONS WAIT, THERE’S MORE! ▸ problem 1. parentheses when using with assign 10> X = catch 4+2. * 1: syntax error before: 'catch' 10> X = (catch 4+2). 6
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  7. ERRORS ANDEXCEPTIONS WAIT, THERE’S MORE! ▸ problem 2. difficult to distinct 11> catch erlang:boat(). 12> catch exit({undef, [{erlang,boat,[]}, {erl_eval,do_apply,5}, {erl_eval,expr,5}, {shell,exprs,6}, {shell,eval_exprs,6}, {shell,eval_loop ,3}]}). {'EXIT',{undef,[{erlang,boat,[]}, {erl_eval,do_apply,5}, {erl_eval,expr,5}, {shell,exprs,6}, {shell,eval_exprs,6}, {shell,eval_loop,3}]}}
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  7. ERRORS ANDEXCEPTIONS WAIT, THERE’S MORE! ▸ problem 3. is it error or…? one_or_two(1) -> return; one_or_two(2) -> throw(return). 13> c(exceptions). {ok,exceptions} 14> catch exceptions:one_or_two(1). return 15> catch exceptions:one_or_two(2). return
• 58.
  7. ERRORS ANDEXCEPTIONS TRY A TRY IN A TREE ▸ lookup/2 %% looks for a given value 'Val' in the tree. has_value(_, {node, 'nil'}) -> false; has_value(Val, {node, {_, Val, _, _}}) -> true; has_value(Val, {node, {_, _, Left, Right}}) -> case has_value(Val, Left) of true -> true; false -> has_value(Val, Right) end.
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  7. ERRORS ANDEXCEPTIONS TRY A TRY IN A TREE ▸ lookup/2
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  7. ERRORS ANDEXCEPTIONS TRY A TRY IN A TREE ▸ lookup/2 - less annoying has_value(Val, Tree) -> try has_value1(Val, Tree) of false -> false catch true -> true end. has_value1(_, {node, 'nil'}) -> false; has_value1(Val, {node, {_, Val, _, _}}) -> throw(true); has_value1(Val, {node, {_, _, Left, Right}}) -> has_value1(Val, Left), has_value1(Val, Right).
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  7. ERRORS ANDEXCEPTIONS TRY A TRY IN A TREE ▸ lookup/2 - less annoying