Language spec

This is a specification for the base language that we build in chapters 2-7. This page is meant as compact reference, not as an introduction.

This specification does not include any of the additional features specified in the project.

Example

The language we’re going to build looks like this:

var n: Int = read_int();
print_int(n);
while n > 1 do {
    if n % 2 == 0 then {
        n = n / 2;
    } else {
        n = 3*n + 1;
    }
    print_int(n);
}

Syntax

This specifies the structure of a syntactically valid program in our language. In other words, this specifies how the parser should work.

An expression is defined recursively as follows, where E, E1, E2, … En represent some other arbitrary expression.

• Integer literal: a whole number between -2⁶³ and 2⁶³ - 1.
• Boolean literal: either true or false.
• Identifier: a word consisting of letters, underscores or digits, but the first character must not be a digit.
• Unary operator: either -E or not E.
• Binary operator: E1 op E2 where op is one of the following: +, -, *, /, %, ==, !=, <, <=, >, >=, and, or, =.
  • Operator = is right-associative.
  • All other operators are left-associative.
  • Precedences are defined below.
• Parentheses: (E), used to override precedence.
• Block: { E1; E2; ...; En } or { E1; E2; ...; En; } (may be empty, last semicolon optional).
  • Semicolons after subexpressions that end in } are optional.
• Untyped variable declaration: var ID = E where ID is an identifier.
• Typed variable declaration: var ID: T = E where ID is an identifier and T is a type expression (defined below).
• If-then conditional: if E1 then E2
• If-then-else conditional: if E1 then E2 else E3
• While-loop: while E1 do E2
• Function call: ID(E1, E2, ..., En) where ID is an identifier

Variable declarations (var ...) are allowed only directly inside blocks ({ ... }) and in top-level expressions.

Precedences:

1. =
2. or
3. and,
4. ==, !=
5. <, <=, >, >=
6. +, -
7. *, /, %
8. Unary - and not
9. All other constructs: literals, identifiers, if, while, var, blocks, parentheses, function calls.

All non-operator expressions such as if, while and function calls must be (syntactically) allowed to be part of other expressions, so e.g. 1 + if true then 2 else 3 must be allowed.

A type expression is defined recursively as follows, where T, T1, T2, … Tn represent some other arbitrary type expression.

• Type variable: an identifier
• Function type expression: (T1, T2, ..., Tn) => T where T1, T2, ..., Tn are the parameter types and T is the result type.

The program consists of a single top-level expression. If the program text has multiple expressions separated by semicolons, they are treated like the contents of a block, and that block becomes the top-level expression. The last expression may be optionally followed by a semicolon.

Arbitrary amounts of whitespace are allowed between tokens. One-line comments starting with # or // are supported.

Semantics

This specifies specify how a program should work.

This works directly as a specification for an interpreter. A compiler should produce a program that behaves the same way.

We don’t specify type-checking here.

A value may be one of the following:

• a 64-bit signed integer (between -2⁶³ and 2⁶³ - 1)
• a boolean true or false
• a built-in function*
• the special value unit, which means ”no meaningful value”

* You don’t need to support any use of function values other than calling them directly.

A context consists of:

• locals: a partial map of identifiers to values
• an optional parent context

A lookup of identifier ID in context C proceeds as follows:

• if ID is defined in C’s locals, return the corresponding value
• otherwise if a lookup of ID in C’s parent succeeds, return that value
• otherwise the lookup fails

An expression is evaluated with a given context. The result of expression evaluation is a value and optionally a modification of the context’s locals.

An expression in a context C is evaluated as follows.

• Literal: return the constant value indicated.
• Identifier ID: look up ID from C, fail if not found.
• Unary operator -E:
  • if the value of E is an integer, return its negation
  • otherwise fail
• Unary operator not E:
  • if the value of E is a boolean, return its negation
  • otherwise fail
• Binary operator E1 = E2:
  • if E1 is an identifier ID, then:
    • define context C' like this:
      • if ID is a local of C, define C' as C
      • otherwise define C' as the closest parent context of C that has ID as a local
      • if C' is still not defined, fail
    • set local ID in C' to the value of E2
  • if E1 is not an identifier, fail
  • return the value of E2
• Binary operator E1 and E2:
  • if E1 evaluates to false, return false and do not evaluate E2
  • otherwise return the value of E2
• Binary operator E1 or E2:
  • if E1 evaluates to true, return true and do not evaluate E2
  • otherwise return the value of E2
• Other binary operators E1 OP E2:
  • look up the value f of OP in C
  • if f is not a function, fail
  • evaluate E1 and then E2
  • call f with the values of E1 and E2 and return the result
• Block { E1; E2; ...; En } or { E1; E2; ...; En; }:
  • create context C' with no locals and parent context C
  • evaluate each expression E1, E2, … in context C'
  • if En is not followed by a semicolon, return the value of En
  • if En is followed by a semicolon, return unit
  • an empty block ({}) is allowed – it returns unit
• Variable declaration var ID = E:
  • if C already has a local ID, fail
  • set local ID in C to the value of E
  • return value unit
• If-then conditional if E1 then E2:
  • if the value of E1 is true, evaluate E2 and return unit
  • if the value of E1 is false, return unit
  • if the value of E1 is something else, fail
  • return unit
• If-then-else conditional if E1 then E2 else E3:
  • if the value of E1 is true, return the value of E2
  • if the value of E1 is false, return the value of E3
  • if the value of E1 is something else, fail
• While-loop while E1 do E2:
  • if the value of E1 is true, evaluate E2, discard its return value, and then start evaluating the while-loop again
  • if the value of E1 is false, return unit
  • if the value of E1 is something else, fail
• Function call E(E1, E2, ..., En):
  • let f be the value of E
  • if f is not a function, fail
  • evaluate each expression E1, E2, …, call f with their values, and return the resutl
  • the implementation is allowed to limit the number of allowed arguments to 6

When evaluating the top-level expression, the initial context has no parent context and has the built-in functions and operators as its locals.

The built-in functions are:

• print_int: prints an integer and a newline to standard output.
• print_bool: prints either true or false and a newline to standard output.
• read_int: reads a single line, including the newline, from standard input, and interprets it as an integer. If the input before the newline contains characters other than digits and a prefix minus, then read_int is allowed to fail or have arbitrary behavior.

The output of a program whose evaluation succeeds consists of the outputs of any built-in print functions that were evaluated, followed by the result of its top-level expression, if it was an integer or a boolean. (If the top-level expression ends in a semicolon, then the result is unit, which is not printed.)