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+---
+language: mercury
+contributors:
+    - ["Julian Fondren", "https://mercury-in.space/"]
+---
+
+Mercury is a strict, pure functional/logic programming language, with
+influences from Prolog, ML, and Haskell.
+
+```prolog
+% Percent sign starts a one-line comment.
+
+    % foo(Bar, Baz)
+    %
+    % Documentation comments are indented before what they describe.
+:- pred foo(bar::in, baz::out) is det.
+
+% All toplevel syntax elements end with a '.' -- a full stop.
+
+% Mercury terminology comes from predicate logic. Very roughly:
+
+% | Mercury               | C                            |
+% |                       |                              |
+% | Goal                  | statement                    |
+% | expression            | expression                   |
+% | predicate rule        | void function                |
+% | function rule         | function                     |
+% | head (of a rule)      | function name and parameters |
+% | body (of a rule)      | function body                |
+% | fact                  | (rule without a body)        |
+% | pred/func declaration | function signature           |
+% | A, B  (conjunction)   | A && B                       |
+% | A ; B (disjunction)   | if (A) {} else if (B) {}     |
+
+% some facts:
+man(socrates).  % "it is a fact that Socrates is a man"
+man(plato).
+man(aristotle).
+
+% a rule:
+mortal(X) :- man(X).  % "It is a rule that X is a mortal if X is a man."
+%            ^^^^^^-- the body of the rule
+%         ^^-- an arrow <--, pointing to the head from the body
+%^^^^^^^^-- the head of the rule
+% this is also a single clause that defines the rule.
+
+% that X is capitalized is how you know it's a variable.
+% that socrates is uncapitalized is how you know it's a term.
+
+% it's an error for 'socrates' to be undefined. It must have a type:
+
+% declarations begin with ':-'
+:- type people
+    --->    socrates
+    ;       plato
+    ;       aristotle
+    ;       hermes.
+    %<--first tab stop (using 4-space tabs)
+            %<--third tab stop (first after --->)
+
+:- pred man(people).  % rules and facts also require types
+
+% a rule's modes tell you how it can be used.
+:- mode man(in) is semidet.  % man(plato) succeeds. man(hermes) fails.
+:- mode man(out) is multi.   % man(X) binds X to one of socrates ; plato ; aristotle
+
+% a semidet predicate is like a test. It doesn't return a value, but
+% it can succeed or fail, triggering backtracking or the other side of
+% a disjunction or conditional.
+
+% 'is semidet' provides the determinism of a mode. Other determinisms:
+% | Can fail? | 0 solutions | 1       | more than 1 |
+% |           |             |         |             |
+% | no        | erroneous   | det     | multi       |
+% | yes       | failure     | semidet | nondet      |
+
+:- pred mortal(people::in) is semidet.  % type/mode in one declaration
+
+% this rule's body consists of two conjunctions: A, B, C
+% this rule is true if A, B, and C are all true.
+% if age(P) returns 16, it fails.
+% if alive(P) fails, it fails.
+:- type voter(people::in) is semidet.
+voter(P) :-
+    alive(P),
+    registered(P, locale(P)),
+    age(P) >= 18.  % age/1 is a function; int.>= is a function used as an operator
+
+% "a P is a voter if it is alive, is registered in P's locale, and if
+% P's age is 18 or older."
+
+% the >= used here is provided by the 'int' module, which isn't
+% imported by default. Mercury has a very small 'Prelude' (the
+% 'builtin' module). You even need to import the 'list' module if
+% you're going to use list literals.
+```
+
+Complete runnable example. File in 'types.m'; compile with 'mmc --make types'.
+
+```prolog
+:- module types.
+:- interface.
+:- import_module io.  % required for io.io types in...
+% main/2 is usually 'det'. threading and exceptions require 'cc_multi'
+:- pred main(io::di, io::uo) is cc_multi.  % program entry point
+:- implementation.
+:- import_module int, float, string, list, bool, map, exception.
+
+% enum.
+:- type days
+    --->    sunday
+    ;       monday
+    ;       tuesday
+    ;       wednesday
+    ;       thursday
+    ;       friday
+    ;       saturday.
+
+% discriminated union, like datatype in ML.
+:- type payment_method
+    --->    cash(int)
+    ;       credit_card(
+                name :: string,         % named fields
+                cc_number :: string,
+                cvv :: int,
+                expiration :: string
+            )
+    ;       crypto(coin_type, wallet, amount).
+
+:- type coin_type
+    --->    etherium
+    ;       monero.  % "other coins are available"
+
+% type aliases.
+:- type wallet == string.
+:- type amount == int.
+
+% !IO is the pair of io.io arguments
+% pass it to anything doing I/O, in order to perform I/O.
+% many otherwise-impure functions can 'attach to the I/O state' by taking !IO
+main(!IO) :-
+    Ints = [
+        3,
+        1 + 1,
+        8 - 1,
+        10 * 2,
+        35 / 5,
+        5 / 2,      % truncating division
+        int.div(5, 2),  % floored division
+        div(5, 2),  % (module is unambiguous due to types)
+        5 `div` 2,  % (any binary function can be an operator with ``)
+        7 `mod` 3,  % modulo of floored division
+        7 `rem` 3,  % remainder of truncating division
+        2 `pow` 4,  % 2 to the 4th power
+        (1 + 3) * 2,    % parens have their usual meaning
+
+        2 >> 3,     % bitwise right shift
+        128 << 3,   % bitwise left shift
+        \ 0,        % bitwise complement
+        5 /\ 1,     % bitwise and
+        5 \/ 1,     % bitwise or
+        5 `xor` 3,  % bitwise xor
+
+        max_int,
+        min_int,
+
+        5 `min` 3,  % ( if 5 > 3 then 3 else 5 )
+        5 `max` 3
+    ],
+    Bools = [
+        yes,
+        no
+        % bools are much less important in Mercury because control flow goes by
+        % semidet goals instead of boolean expressions.
+    ],
+    Strings = [
+        "this is a string",
+        "strings can have "" embedded doublequotes via doubling",
+        "strings support \u4F60\u597D the usual escapes\n",
+        % no implicit concatenation of strings: "concat:" "together"
+        "but you can " ++ " use the string.++ operator",
+
+        % second param is a list(string.poly_type)
+        % s/1 is a function that takes a string and returns a poly_type
+        % i/1 takes an int. f/1 takes a float. c/1 takes a char.
+        string.format("Hello, %d'th %s\n", [i(45), s("World")])
+    ],
+
+    % start with purely functional types like 'map' and 'list'!
+    % arrays and hash tables are available too, but using them
+    % requires knowing a lot more about Mercury
+    get_map1(Map1),
+    get_map2(Map2),
+
+    % list.foldl has *many* variations
+    % this one calls io.print_line(X, !IO) for each X of the list
+    foldl(io.print_line, Ints, !IO),
+    foldl(io.print_line, Bools, !IO),
+    foldl(io.print_line, Strings, !IO),
+    io.print_line(Map1, !IO),
+    % ( if Cond then ThenGoal else ElseGoal )
+    % I/O not allowed in Cond: I/O isn't allowed to fail!
+    ( if Map2^elem(42) = Elem then
+        io.print_line(Elem, !IO)
+    else % always required
+        true  % do nothing, successfully (vs. 'fail')
+    ),
+
+    % exception handling:
+    ( try [io(!IO)] ( % io/1 param required or no I/O allowed here
+        io.print_line(received(cash(1234)), !IO),
+        io.print_line(received(crypto(monero, "invalid", 123)), !IO)
+    ) then
+        io.write_string("all payments accepted\n", !IO) % never reached
+    catch "monero not yet supported" -> % extremely specific catch!
+        io.write_string("monero payment failed\n", !IO)
+    ).
+
+:- pred get_map1(map(string, int)::out) is det.
+get_map1(!:Map) :-  % !:Map in the head is the final (free, unbound) Map
+    !:Map = init,   % !:Map in the body is the next Map
+    det_insert("hello", 1, !Map),  % pair of Map vars
+    det_insert("world", 2, !Map),
+
+    % debug print of current (bound) Map
+    % other [Params] can make it optional per runtime or compiletime flags
+    trace [io(!IO)] (io.print_line(!.Map, !IO)),
+
+    det_insert_from_corresponding_lists(K, V, !Map),
+    % this code is reordered so that K and V and defined prior to their use
+    K = ["more", "words", "here"],
+    V = [3, 4, 5].
+
+:- pred get_map2(map(int, bool)::out) is det.
+get_map2(Map) :-
+    det_insert(42, yes, map.init, Map).
+
+:- func received(payment_method) = string.
+received(cash(N)) = string.format("received %d dollars", [i(N)]).
+received(credit_card(_, _, _, _)) = "received credit card".  % _ is throwaway
+received(crypto(Type, _Wallet, Amount)) = S :-  % _Wallet is named throwaway
+    ( % case/switch structure
+        Type = etherium,
+        S = string.format("receiving %d ETH", [i(Amount)])
+    ;
+        Type = monero,
+        throw("monero not yet supported")  % exception with string as payload
+    ).
+```
+
+## That was quick! Want more?
+
+### More Tutorials
+
+* [Mercury Tutorial](https://mercurylang.org/documentation/papers/book.pdf) (pdf link) - a more traditional tutorial with a more relaxed pace
+* [Mercury Crash Course](https://mercury-in.space/crash.html) - a dense example-driven tutorial with Q&A format
+* [Github Wiki Tutorial](https://github.com/Mercury-Language/mercury/wiki/Tutorial)
+* [Getting Started with Mercury](https://bluishcoder.co.nz/2019/06/23/getting-started-with-mercury.html) - installation and your first steps
+  
+### Documentation
+
+* Language manual, user's guide, and library reference are all at
+  [mercurylang.org](https://mercurylang.org/documentation/documentation.html)