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