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diff --git a/mercury.html.markdown b/mercury.html.markdown new file mode 100644 index 00000000..76c7d1ca --- /dev/null +++ b/mercury.html.markdown @@ -0,0 +1,263 @@ +--- +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) |