diff options
Diffstat (limited to 'erlang.html.markdown')
| -rw-r--r-- | erlang.html.markdown | 41 |
1 files changed, 26 insertions, 15 deletions
diff --git a/erlang.html.markdown b/erlang.html.markdown index 208f31e4..d7040245 100644 --- a/erlang.html.markdown +++ b/erlang.html.markdown @@ -6,7 +6,7 @@ filename: learnerlang.erl --- ```erlang -% Percent sign start a one-line comment. +% Percent sign starts an one-line comment. %% Two percent characters shall be used to comment functions. @@ -17,7 +17,7 @@ filename: learnerlang.erl % patterns. % Periods (`.`) (followed by whitespace) separate entire functions and % expressions in the shell. -% Semicolons (`;`) separate clauses. We find clauses in several contexts: in kn +% Semicolons (`;`) separate clauses. We find clauses in several contexts: % function definitions and in `case`, `if`, `try..catch` and `receive` % expressions. @@ -26,8 +26,10 @@ filename: learnerlang.erl %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Num = 42. % All variable names must start with an uppercase letter. + % Erlang has single assignment variables, if you try to assign a different value % to the variable `Num`, you’ll get an error. +Num = 43. % ** exception error: no match of right hand side value 43 % In most languages, `=` denotes an assignment statement. In Erlang, however, % `=` denotes a pattern matching operation. `Lhs = Rhs` really means this: @@ -42,6 +44,11 @@ Pi = 3.14159. % start with lowercase letters, followed by a sequence of alphanumeric % characters or the underscore (`_`) or at (`@`) sign. Hello = hello. +OtherNode = example@node. + +% Atoms with non alphanumeric values can be written by enclosing the atoms +% with apostrophes. +AtomWithSpace = 'some atom with space'. % Tuples are similar to structs in C. Point = {point, 10, 45}. @@ -60,15 +67,15 @@ Person = {person, {name, {first, joe}, {last, armstrong}}, {footsize, 42}}. % We create a list by enclosing the list elements in square brackets and % separating them with commas. % The individual elements of a list can be of any type. -% The first element of a list the head of the list. If you imagine removing the +% The first element of a list is the head of the list. If you imagine removing the % head from the list, what’s left is called the tail of the list. ThingsToBuy = [{apples, 10}, {pears, 6}, {milk, 3}]. -% If `T` is a list, then `[H|T]` is also a list, with head H and tail T. +% If `T` is a list, then `[H|T]` is also a list, with head `H` and tail `T`. % The vertical bar (`|`) separates the head of a list from its tail. % `[]` is the empty list. % We can extract elements from a list with a pattern matching operation. If we -% have the nonempty list `L`, then the expression `[X|Y] = L`, where `X` and `Y` +% have a nonempty list `L`, then the expression `[X|Y] = L`, where `X` and `Y` % are unbound variables, will extract the head of the list into `X` and the tail % of the list into `Y`. [FirstThing|OtherThingsToBuy] = ThingsToBuy. @@ -78,6 +85,7 @@ ThingsToBuy = [{apples, 10}, {pears, 6}, {milk, 3}]. % There are no strings in Erlang. Strings are really just lists of integers. % Strings are enclosed in double quotation marks (`"`). Name = "Hello". +[72, 101, 108, 108, 111] = "Hello". %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% @@ -89,9 +97,9 @@ Name = "Hello". % Modules must be compiled before the code can be run. A compiled module has the % extension `.beam`. -module(geometry). --export([area/1]). +-export([area/1]). % the list of functions exported from the module. -% The function area consists of two clauses. The clauses are separated by a +% The function `area` consists of two clauses. The clauses are separated by a % semicolon, and the final clause is terminated by dot-whitespace. % Each clause has a head and a body; the head consists of a function name % followed by a pattern (in parentheses), and the body consists of a sequence of @@ -109,17 +117,17 @@ c(geometry). % {ok,geometry} geometry:area({rectangle, 10, 5}). % 50 geometry:area({circle, 1.4}). % 6.15752 -% In Erlang, two functions with the same name and different arity in the same -% module represent entirely different functions. +% In Erlang, two functions with the same name and different arity (number of arguments) +% in the same module represent entirely different functions. -module(lib_misc). --export([sum/1]). +-export([sum/1]). % export function `sum` of arity 1 accepting one argument: list of integers. sum(L) -> sum(L, 0). sum([], N) -> N; sum([H|T], N) -> sum(T, H+N). -% Funs are "anonymous" functions. They are called this because they have no -% name. -Double = fun(X) -> 2*X end. +% Funs are "anonymous" functions. They are called this way because they have no +% name. However they can be assigned to variables. +Double = fun(X) -> 2*X end. % `Double` points to an anonymous function with handle: #Fun<erl_eval.6.17052888> Double(2). % 4 % Functions accept funs as their arguments and can return funs. @@ -133,6 +141,8 @@ Triple(5). % 15 % from the list `L`." L = [1,2,3,4,5]. [2*X || X <- L]. % [2,4,6,8,10] +% A list comprehension can have generators and filters which select subset of the generated values. +EvenNumbers = [N || N <- [1, 2, 3, 4], N rem 2 == 0]. % [2, 4] % Guards are constructs that we can use to increase the power of pattern % matching. Using guards, we can perform simple tests and comparisons on the @@ -181,7 +191,7 @@ X2 = X1#todo{status = done}. % #todo{status = done,who = joe,text = "Fix errata in book"} % `case` expressions. -% `filter` returns a list of all those elements `X` in `L` for which `P(X)` is +% `filter` returns a list of all elements `X` in a list `L` for which `P(X)` is % true. filter(P, [H|T]) -> case P(H) of @@ -189,6 +199,7 @@ filter(P, [H|T]) -> false -> filter(P, T) end; filter(P, []) -> []. +filter(fun(X) -> X rem 2 == 0, [1, 2, 3, 4]). % [2, 4] % `if` expressions. max(X, Y) -> @@ -198,7 +209,7 @@ max(X, Y) -> true -> nil; end. -% Warning: at least one of the guards in the if expression must evaluate to true; +% Warning: at least one of the guards in the `if` expression must evaluate to true; % otherwise, an exception will be raised. |