path: root/mercury.html.markdown

---
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.

```mercury
% 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'.

```mercury
:- 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)