diff options
Diffstat (limited to 'erlang.html.markdown')
| -rw-r--r-- | erlang.html.markdown | 96 |
1 files changed, 54 insertions, 42 deletions
diff --git a/erlang.html.markdown b/erlang.html.markdown index a7390c3e..a3b571d1 100644 --- a/erlang.html.markdown +++ b/erlang.html.markdown @@ -18,7 +18,7 @@ filename: learnerlang.erl % Periods (`.`) (followed by whitespace) separate entire functions and % expressions in the shell. % Semicolons (`;`) separate clauses. We find clauses in several contexts: -% function definitions and in `case`, `if`, `try..catch` and `receive` +% function definitions and in `case`, `if`, `try..catch`, and `receive` % expressions. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% @@ -27,20 +27,20 @@ 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. +% 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: -% evaluate the right side (Rhs), and then match the result against the pattern -% on the left side (Lhs). +% `=` denotes a pattern-matching operation. `Lhs = Rhs` really means this: +% evaluate the right side (`Rhs`), and then match the result against the +% pattern on the left side (`Lhs`). Num = 7 * 6. -% Floating point number. +% Floating-point number. Pi = 3.14159. -% Atoms, are used to represent different non-numerical constant values. Atoms +% Atoms are used to represent different non-numerical constant values. Atoms % start with lowercase letters, followed by a sequence of alphanumeric % characters or the underscore (`_`) or at (`@`) sign. Hello = hello. @@ -53,34 +53,34 @@ AtomWithSpace = 'some atom with space'. % Tuples are similar to structs in C. Point = {point, 10, 45}. -% If we want to extract some values from a tuple, we use the pattern matching +% If we want to extract some values from a tuple, we use the pattern-matching % operator `=`. {point, X, Y} = Point. % X = 10, Y = 45 % We can use `_` as a placeholder for variables that we’re not interested in. % The symbol `_` is called an anonymous variable. Unlike regular variables, -% several occurrences of _ in the same pattern don’t have to bind to the same -% value. +% several occurrences of `_` in the same pattern don’t have to bind to the +% same value. Person = {person, {name, {first, joe}, {last, armstrong}}, {footsize, 42}}. {_, {_, {_, Who}, _}, _} = Person. % Who = joe % 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 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. +% 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`. % 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 +% We can extract elements from a list with a pattern-matching operation. If we % 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. % FirstThing = {apples, 10} -% OtherThingsToBuy = {pears, 6}, {milk, 3} +% OtherThingsToBuy = [{pears, 6}, {milk, 3}] % There are no strings in Erlang. Strings are really just lists of integers. % Strings are enclosed in double quotation marks (`"`). @@ -117,17 +117,19 @@ 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 (number of arguments) -% 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 function `sum` of arity 1 accepting one argument: list of integers. +-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 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> +% 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. @@ -140,8 +142,9 @@ Triple(5). % 15 % The notation `[F(X) || X <- L]` means "the list of `F(X)` where `X` is taken % 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. +[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 @@ -155,17 +158,24 @@ max(X, Y) -> Y. % A guard is a series of guard expressions, separated by commas (`,`). % The guard `GuardExpr1, GuardExpr2, ..., GuardExprN` is true if all the guard -% expressions `GuardExpr1, GuardExpr2, ...` evaluate to true. +% expressions `GuardExpr1`, `GuardExpr2`, ..., `GuardExprN` evaluate to `true`. is_cat(A) when is_atom(A), A =:= cat -> true; is_cat(A) -> false. is_dog(A) when is_atom(A), A =:= dog -> true; is_dog(A) -> false. -% A `guard sequence` is either a single guard or a series of guards, separated -%by semicolons (`;`). The guard sequence `G1; G2; ...; Gn` is true if at least -% one of the guards `G1, G2, ...` evaluates to true. -is_pet(A) when is_dog(A); is_cat(A) -> true; -is_pet(A) -> false. +% A guard sequence is either a single guard or a series of guards, separated +% by semicolons (`;`). The guard sequence `G1; G2; ...; Gn` is true if at +% least one of the guards `G1`, `G2`, ..., `Gn` evaluates to `true`. +is_pet(A) when is_atom(A), (A =:= dog) or (A =:= cat) -> true; +is_pet(A) -> false. + +% Warning: not all valid Erlang expressions can be used as guard expressions; +% in particular, our `is_cat` and `is_dog` functions cannot be used within the +% guard sequence in `is_pet`'s definition. For a description of the +% expressions allowed in guard sequences, refer to this +% [section](http://erlang.org/doc/reference_manual/expressions.html#id81912) +% of the Erlang reference manual. % Records provide a method for associating a name with a particular element in a % tuple. @@ -188,7 +198,7 @@ X = #todo{}. X1 = #todo{status = urgent, text = "Fix errata in book"}. % #todo{status = urgent, who = joe, text = "Fix errata in book"} X2 = X1#todo{status = done}. -% #todo{status = done,who = joe,text = "Fix errata in book"} +% #todo{status = done, who = joe, text = "Fix errata in book"} % `case` expressions. % `filter` returns a list of all elements `X` in a list `L` for which `P(X)` is @@ -209,8 +219,8 @@ max(X, Y) -> true -> nil end. -% Warning: at least one of the guards in the `if` expression must evaluate to true; -% otherwise, an exception will be raised. +% Warning: at least one of the guards in the `if` expression must evaluate to +% `true`; otherwise, an exception will be raised. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% @@ -218,7 +228,7 @@ max(X, Y) -> %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% % Exceptions are raised by the system when internal errors are encountered or -% explicitly in code by calling `throw(Exception)`, `exit(Exception)` or +% explicitly in code by calling `throw(Exception)`, `exit(Exception)`, or % `erlang:error(Exception)`. generate_exception(1) -> a; generate_exception(2) -> throw(a); @@ -227,7 +237,7 @@ generate_exception(4) -> {'EXIT', a}; generate_exception(5) -> erlang:error(a). % Erlang has two methods of catching an exception. One is to enclose the call to -% the function, which raised the exception within a `try...catch` expression. +% the function that raises the exception within a `try...catch` expression. catcher(N) -> try generate_exception(N) of Val -> {N, normal, Val} @@ -241,23 +251,24 @@ catcher(N) -> % exception, it is converted into a tuple that describes the error. catcher(N) -> catch generate_exception(N). -%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% + +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %% 4. Concurrency %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% % Erlang relies on the actor model for concurrency. All we need to write -% concurrent programs in erlang are three primitives: spawning processes, +% concurrent programs in Erlang are three primitives: spawning processes, % sending messages and receiving messages. -% To start a new process we use the `spawn` function, which takes a function +% To start a new process, we use the `spawn` function, which takes a function % as argument. F = fun() -> 2 + 2 end. % #Fun<erl_eval.20.67289768> spawn(F). % <0.44.0> -% `spawn` returns a pid (process identifier), you can use this pid to send -% messages to the process. To do message passing we use the `!` operator. -% For all of this to be useful we need to be able to receive messages. This is +% `spawn` returns a pid (process identifier); you can use this pid to send +% messages to the process. To do message passing, we use the `!` operator. +% For all of this to be useful, we need to be able to receive messages. This is % achieved with the `receive` mechanism: -module(calculateGeometry). @@ -272,12 +283,13 @@ calculateArea() -> io:format("We can only calculate area of rectangles or circles.") end. -% Compile the module and create a process that evaluates `calculateArea` in the shell +% Compile the module and create a process that evaluates `calculateArea` in the +% shell. c(calculateGeometry). CalculateArea = spawn(calculateGeometry, calculateArea, []). CalculateArea ! {circle, 2}. % 12.56000000000000049738 -% The shell is also a process, you can use `self` to get the current pid +% The shell is also a process; you can use `self` to get the current pid. self(). % <0.41.0> ``` |