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| language | contributors | |||
|---|---|---|---|---|
| Mercury |
|
Mercury is a strict, pure functional/logic programming language, with influences from Prolog, ML, and Haskell.
% 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'.
:- 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 (pdf link) - a more traditional tutorial with a more relaxed pace
- Mercury Crash Course - a dense example-driven tutorial with Q&A format
- GitHub Wiki Tutorial
- Getting Started with Mercury - installation and your first steps
Documentation
- Language manual, user's guide, and library reference are all at mercurylang.org