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Env

type Env = @moonbitlang/core/immut/hashmap.HashMap
HashMap[String, Value]

An environment: ρ. Immutable, because a closure captures one by value.

DefClosure

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pub(all) struct DefClosure {
  env : @moonbitlang/core/immut/hashmap.HashMap
HashMap[String, Value]
  region : Array[@marianoguerra/pure-py/ast.Stmt
Stmt]
  index : Int
  in_module : String
} derive(@moonbitlang/core/debug.Debug
Debug)

LamClosure

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pub(all) struct LamClosure {
  env : @moonbitlang/core/immut/hashmap.HashMap
HashMap[String, Value]
  params : Array[String]
  body : @marianoguerra/pure-py/ast.Expr
Expr
  in_module : String
} derive(@moonbitlang/core/debug.Debug
Debug)

in_module is not part of LamΓ: the spec's closures carry an environment and a body, and nothing about where they were written. It is here because a position without a file is a lie the moment a program has two modules -- a helper.py function that aborts on its line 4 would be read as line 4 of __main__. The evaluator's import stack cannot supply it, because during a call it names the module that IMPORTED the callee, not the one that defined it. Only the closure knows.

MatchResult

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pub(all) enum MatchResult {
  Match(@moonbitlang/core/immut/hashmap.HashMap
HashMap[String, Value])
  NoMatch
  MatchStuck(String)
} derive(@moonbitlang/core/debug.Debug
Debug)

Whether a pattern matched, and what it bound.

MatchResult::then

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fn MatchResult::then(self : MatchResult, other : MatchResult) -> MatchResult

m ⊗ m': a match result composes by union of bindings, and no-match is absorbing.

Outcome

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pub(all) enum Outcome {
  Val(Value)
  Aborts(Termination)
  Stuck(String)
} derive(@moonbitlang/core/debug.Debug
Debug)

The outcome of evaluating an expression: a value, an abort, or an operation the semantics leaves undefined.

Primitive

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pub(all) enum Primitive {
  Print
  Len
  Range
  Exit
  Sqrt
  Exp
  Log
  Sin
  Cos
  Tan
  MathFloor
  MathCeil
  Opaque(String)
  Foreign(String)
} derive(Eq, @moonbitlang/core/debug.Debug
Debug)

The predefined functions of Figure 2.7.

Opaque is for the three members that are values only so that importing them binds something: typing.Any, typing.Callable and dataclasses.dataclass are never called.

StmtResult

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pub(all) enum StmtResult {
  Assigns(@moonbitlang/core/immut/hashmap.HashMap
HashMap[String, Value])
  Returns(Value)
  ResultAborts(Termination)
  ResultStuck(String)
} derive(@moonbitlang/core/debug.Debug
Debug)

The result of evaluating a statement.

StmtResult::outcome

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fn StmtResult::outcome(self : StmtResult) -> Outcome

outcome(r): a statement's result read as the outcome of calling it.

StmtResult::then

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fn StmtResult::then(self : StmtResult, other : StmtResult) -> StmtResult

Sequential composition of results: assigns ρ ⊗ r.

Termination

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pub(all) enum Termination {
  TypeError
  IndexError
  KeyError
  ZeroDivisionError
  AttributeError
  AssertionError(String?)
  SystemExit(@moonbitlang/core/bigint.BigInt
BigInt)
} derive(Eq, @moonbitlang/core/debug.Debug
Debug)

How a run ends. Every kind but SystemExit is named after the exception the same program raises under Python.

Termination::name

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fn Termination::name(self : Termination) -> String

The name Python prints for a termination, and the message after it.

Termination::to_text

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fn Termination::to_text(self : Termination) -> String

AssertionError: message when there is one, the bare name otherwise -- the last line of the traceback Python prints.

Value

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pub(all) enum Value {
  None
  Bool(Bool)
  Int(@moonbitlang/core/bigint.BigInt
BigInt)
  Float(Double)
  Str(String)
  List(Array[Value])
  Tuple(Array[Value])
  Dict(Array[(String, Value)])
  Lam(LamClosure)
  Def(DefClosure)
  Obj(@marianoguerra/pure-py/context.ClassEntry
ClassEntry, @moonbitlang/core/immut/hashmap.HashMap
HashMap[String, Value])
  ModStub(String)
  Mod(String, @moonbitlang/core/immut/hashmap.HashMap
HashMap[String, Value])
  Class(@marianoguerra/pure-py/context.ClassEntry
ClassEntry)
  Prim(Primitive)
} derive(@moonbitlang/core/debug.Debug
Debug)

Runtime values, environments and evaluation results: Figure 4.1.

Two things about this type are the whole shape of the language:

Nothing is mutable. A list, a tuple, a dictionary and an object are built once and never changed, so an environment can be shared by every closure that captures it and nothing has to be copied defensively.
Stuck is a result, not a crash. The semantics leaves some operations undefined, and Chapter 1 allows an implementation either to abort or to produce Python's answer. This one aborts, with the operation named, so that the conformance suite's dynamically excluded bucket is CHECKED rather than merely tolerated.

Value::kind_name

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fn Value::kind_name(self : Value) -> String

What kind of value this is, for a message naming an undefined operation.

Value::repr

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fn Value::repr(self : Value) -> String?

Value::str

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fn Value::str(self : Value) -> String?

Python's str and repr of a value.

The two differ in exactly one place -- a string is itself under str and quoted under repr -- and print uses str while a container prints its elements with repr. Everything else is the same function.

A closure, a module, a class or a primitive has no printable form here. Python prints one with an address in it, which no implementation can reproduce, so printing one is an undefined operation and is reported as such rather than invented.

big_to_double

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fn big_to_double(n : @moonbitlang/core/bigint.BigInt
BigInt) -> Double

A BigInt as a double, correctly rounded -- which is what Python's float(n) gives. MoonBit's BigInt has no conversion, so it goes through the decimal text, which parse_double rounds correctly.

binop

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fn binop(op : @marianoguerra/pure-py/ast.Operator
Operator, a : Value, b : Value) -> Outcome

The operators: ⊙̂(v, v') and ⊖̂(v).

Arithmetic is Python's, with three things this port has to supply that MoonBit does not:

// and % floor. MoonBit's / and % on BigInt and Int64 truncate toward zero; Python's floor toward negative infinity, so the remainder takes the sign of the DIVISOR: -7 // 2 is -4 and 7 % -2 is -1.
bool is not a number. True + 1, True < 2 and -False are UNDEFINED, even though Python accepts all three. Undefined and not TypeError: a termination kind is "named after the exception the same program raises under Python in the same circumstances" (operational-semantics.tex), and Python raises nothing for True + 1 -- it says 2. This is the same rule that makes True == 1 undefined, and the conformance suite has a test for that.
An integer and a float compare exactly, not by converting the integer, so a large integer is not made equal to the double nearest it.

+ and * are tried over sequences first, because they are not arithmetic there. The spec's arithmetic table is a \todo reading "Arithmetic, as in Python ... aborts TypeError where an operand is not a number", and the two halves disagree about "a" + "b": the sentence was written for 1 + "a", which the conformance suite tests and which IS a TypeError in Python. Concatenation is not, and CPython is the oracle for what run prints, so "as in Python" is the half that decides.

compare

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fn compare(op : @marianoguerra/pure-py/ast.CmpOp
CmpOp, a : Value, b : Value) -> Outcome

==, !=, in, not in and the four orderings.

contains

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fn contains(haystack : Value, needle : Value) -> Bool?

Whether needle is in haystack: an element of a list or tuple, a key of a dictionary, a substring of a string.

contains_elems

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fn contains_elems(xs : Array[Value], needle : Value) -> Bool?

elems

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fn elems(v : Value) -> Array[Value]?

The elements of a list, a tuple or a string. A string's elements are its CODE POINTS, which is what Python iterates.

empty_env

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fn empty_env() -> @moonbitlang/core/immut/hashmap.HashMap
HashMap[String, Value]

entries

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fn entries(base : Array[(String, Value)], more : Array[(String, Value)]) -> Array[(String, Value)]

entries(δ, δ'): the left entries extended by the right ones in order.

env_of

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fn env_of(entries : Array[(String, Value)]) -> @moonbitlang/core/immut/hashmap.HashMap
HashMap[String, Value]

eq

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fn eq(a : Value, b : Value) -> Bool?

The functions on values of Annex A.3: equality, membership, elements, iteration, subscripting and the dictionary builders.

Equality is PARTIAL, and the order in which elements are compared is therefore observable: eq short-circuits at the first pair that decides the answer, so an undefined comparison later in a sequence does not make the whole comparison undefined. [1, "a"] == [1, 3] is stuck and [1, "a"] == [2, 3] is False, and the difference is deliberate.

Every function that can be undefined returns a Bool? or an Outcome; None and Stuck mean "no rule", not "false".

eq_elems

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fn eq_elems(xs : Array[Value], ys : Array[Value]) -> Bool?

Elementwise equality, stopping at the first pair that decides it.

exact_integer

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fn exact_integer(d : Double) -> @moonbitlang/core/bigint.BigInt
BigInt?

The integer a double exactly equals, or None when it is not one.

Read off the bit pattern rather than through decimal text, so it is exact at every magnitude: a double is mantissa * 2^exponent, and when it is integral the shift below loses nothing.

extend_env

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fn extend_env(a : @moonbitlang/core/immut/hashmap.HashMap
HashMap[String, Value], b : @moonbitlang/core/immut/hashmap.HashMap
HashMap[String, Value]) -> @moonbitlang/core/immut/hashmap.HashMap
HashMap[String, Value]

ρ ⊲ ρ': extension, which differs from override only on modules -- a loaded module is preferred to a stub of the same module, and two loadings of one module merge member by member.

extend_value

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fn extend_value(a : Value, b : Value) -> Value

float_mod

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fn float_mod(x : Double, y : Double) -> Double

Python's % on floats: the result takes the sign of the divisor.

floor_div

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fn floor_div(x : @moonbitlang/core/bigint.BigInt
BigInt, y : @moonbitlang/core/bigint.BigInt
BigInt) -> @moonbitlang/core/bigint.BigInt
BigInt

Python's // on integers: floor, not truncation.

floor_mod

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fn floor_mod(x : @moonbitlang/core/bigint.BigInt
BigInt, y : @moonbitlang/core/bigint.BigInt
BigInt) -> @moonbitlang/core/bigint.BigInt
BigInt

Python's % on integers: the remainder takes the sign of the divisor.

getitem

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fn getitem(v : Value, key : Value) -> Outcome

Subscripting: a dictionary by a string key, a sequence by an integer index counting from the end when negative.

host_fn

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fn host_fn(name : String) -> Value

A function the host supplies, as a value the guest can call.

The host puts one of these in a module (@eval.HostModule) and answers calls to it by name.

iter

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fn iter(v : Value) -> Array[Value]?

What a generator draws from a value: the KEYS of a dictionary, and otherwise its elements.

order

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fn order(a : Value, b : Value) -> Int?

Three-way comparison, None where the two values have no order.

Numbers with numbers, strings by code point, lists with lists and tuples with tuples lexicographically. A nan anywhere makes the pair unordered, which is how Python's comparisons all come out False.

override_env

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fn override_env(a : @moonbitlang/core/immut/hashmap.HashMap
HashMap[String, Value], b : @moonbitlang/core/immut/hashmap.HashMap
HashMap[String, Value]) -> @moonbitlang/core/immut/hashmap.HashMap
HashMap[String, Value]

ρ ⊗ ρ': the right-hand side wins outright.

unop

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fn unop(op : @marianoguerra/pure-py/ast.UnaryOp
UnaryOp, v : Value) -> Outcome

not, unary + and unary -.

update

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fn update(entries : Array[(String, Value)], key : String, v : Value) -> Array[(String, Value)]

update(δ, w, v): the entries with w bound to v, in place if it was already there and appended otherwise.

Source Files

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#Env

#DefClosure

#LamClosure

#MatchResult

#MatchResult::then

#Outcome

#Primitive

#StmtResult

#StmtResult::outcome

#StmtResult::then

#Termination

#Termination::name

#Termination::to_text

#Value

#Value::kind_name

#Value::repr

#Value::str

#big_to_double

#binop

#compare

#contains

#contains_elems

#elems

#empty_env

#entries

#env_of

#eq

#eq_elems

#exact_integer

#extend_env

#extend_value

#float_mod

#floor_div

#floor_mod

#getitem

#host_fn

#iter

#order

#override_env

#unop

#update

Source Files

Env

DefClosure

LamClosure

MatchResult

MatchResult::then

Outcome

Primitive

StmtResult

StmtResult::outcome

StmtResult::then

Termination

Termination::name

Termination::to_text

Value

Value::kind_name

Value::repr

Value::str

big_to_double

binop

compare

contains

contains_elems

elems

empty_env

entries

env_of

eq

eq_elems

exact_integer

extend_env

extend_value

float_mod

floor_div

floor_mod

getitem

host_fn

iter

order

override_env

unop

update