Lab 01 - Interpreter

Welcome to the Compiler course 2024!

In this first lab, you will implement an interpreter for the Alpine language, the language for which we will write the entire compiler during the semester.

This lab is due on Friday 1st of March at 7pm.

Preparing the environment

We will use the following tools, make sure to install them before starting the lab:

scala, sbt, …: The whole project will be done in Scala. Hence, you need to have Scala and sbt installed on your machine. You can see a tutorial for installing the toolchain here.
If you use Visual Studio Code, you can install the Metals extension to have a better experience with Scala. You can find an up-to-date tutorial to install it here.
If you use IntelliJ, you can install the Scala plugin to have a better experience with Scala. You can find an up-to-date tutorial to install it here.
If you use another editor, feel free to check Metals' documentation to see if there is a plugin for your editor.

Common issues

No autocompletion on Visual Studio Code (VSC)

If the autocompletion does not work, check the following on VSC:

Metals is indeed installed.
build.sbt file is at the root of your project. This means that the folder that you open inside Visual Studio Code is the folder that contains the build.sbt file.
Make sure to import the build inside Metals. You can do it by either pressing Ctrl+Shift+P and typing Import build or clicking on the "Metals" tab in the left panel and then clicking on the "Import build" under the "Build commands" panel.
- A loading icon should appear at the bottom-right and should disappear after a few seconds. If it does not, you can click on the icon and see what is the issue.
- If it does not work, you can select "Clean and restart build server" and click on "Reset workspace" in the pop up.
- If the issue persists, you can try to restart Visual Studio Code.
- If it still does not work, you can see the logs by clicking on the "Check logs" button under the "Help and feedback" pane in the "Metals" tab to inspect it.

Obtaining the lab files

To get the lab files, clone this repository. To do so, open a terminal and run the following command:

$ git clone https://github.com/epfl-lara/compiler2024-labs-public.git

Then the interpreter folder contains this week lab files.

Submit your work

We are using an automatic grading infrastructure to grade your work. You will submit your work on Moodle and automatically receive your grade after a few minutes. To submit your work, go to this week assignment on Moodle and upload the following file:

src/main/scala/alpine/evaluation/Interpreter.scala

When you submit your work, it will create a "Draft" submission. You then have to click on the "Submit" button to submit your work. DO NOT FORGET TO DO THIS STEP! You will receive an email confirming that your work has been submitted. If you do not receive this email, it means that your work has not been properly submitted.

General idea of the project

Let's recall the global idea of a simple compiler's pipeline:

Source code -> Lexer -> Parser -> Type checking -> Assembly generation

To recap,

The Lexer is responsible for transforming the source code into a list of tokens (words, numbers, symbols, etc.). It is the first step of the pipeline.
The Parser is responsible for transforming the list of tokens into an abstract syntax tree (AST). It is the second step of the pipeline.
The Type checking is responsible for checking that the program is well-typed. It is the third step of the pipeline.
The Assembly generation is responsible for generating the machine code. It is the last step of the pipeline.

This week, we will not do a compiler but an interpreter. The difference is that we will not generate machine code but execute the program directly. Hence, the pipeline is simplified:

Source code -> Lexer -> Parser -> Type checker -> Interpreter

Here, the Interpreter is responsible for executing the program. It is the last step of the pipeline. Note that the interpreter interprets a TypedProgram and not a Program. This means that the type checker is run on the AST to solve references to types and variables.

Lab formalities

In this lab, you will modify only the evaluation/Interpreter.scala file. You can obviously check other files to have a better understanding of the project, but you should not modify them.

To run the program, you can use the following SBT command (inside the sbt terminal launched from the root of the project, i.e., where the build.sbt file is located):

run -i <input_file>

This will run the interpreter on the file <input_file> (an Alpine source file).

We provide you a test suite (comprising unit, integration, and end-to-end tests) to check your work. To run it use the following SBT command:

test

These also are the tests on which your work will be graded.

Alpine language

You can find a description of the language inside the language_desc.md/'language_desc.html` file. It contains a small overview of the language, its syntax, and its semantics.

The AST

Let's recall the pipeline seen above. The Parser is responsible for transforming the list of tokens into an Abstract Syntax Tree (AST).

The interpreter will traverse the AST produced by the parser and execute the program.

The AST is a tree that represents the structure of the program. It is a data structure that is used to represent the program in a way that is easy to manipulate. For instance, the following program:

let x = 1

is represented by the following AST:

IArray(
  Binding(
    x, // identifier
    None, // explicit type on the definition
    Some( // initialisation content (i.e. the 1), if exists
      IntegerLiteral(
        1, // literal value
        <input>:1:9-1:10 // position of the literal
      )
    ), 
    <input>:1:1-1:10 // position of the binding
  )
)

If you have taken the CS-214 Software Construction course: yes, it looks similar to the evaluator we have seen, though more complete (and therefore complex).

You can find the definitions of all the types of the AST in the alpine.ast.Trees file (src/main/scala/alpine/ast/Trees.scala file.)

Implementing the interpreter

You will implement the interpreter in the alpine.evaluation.Interpreter class (src/main/scala/alpine/evaluation/Interpreter.scala file.)

Using SBT

To run files with the interpreter, you can use the following SBT command (inside the sbt terminal launched from the root of the project, i.e., where the build.sbt file is located):

run -i <input_file>

To test your code, you can use the following SBT command:

test

Step 0: Entrypoint (provided)

The entrypoint of an Alpine program is its main variable (i.e. variable called main.) The interpreter has to find the expression contained in the main variable and interpret it.

Inside the interpreter class, you will find the run method that is the entry point of the interpreter. The variable of type TypedProgram contains the AST of the Alpine program to interpret.

/** Evaluates the entry point of the program. */
def run(): Int =
  val e = syntax.entry.getOrElse(throw Panic("no entry point"))
  try
    e.visit(this)(using Context())
    0
  catch case e: Interpreter.Exit =>
    e.status

src/main/scala/alpine/evaluation/Interpreter.scala

Note that the run method is already implemented. It calls the visit method on the expression contained in the main variable which can be retrieved by calling the entry method on the TypedProgram instance.

The general idea is that the interpreter will successively visit the nodes of the AST and execute the program while visiting.

Step 1: Parenthesized expressions

A parenthesized expression is an expression that is enclosed in parentheses. In Alpine, a parenthesized expression is written as (<expression>). The interpreter should evaluate parenthesized expressions when it visits a ParenthesizedExpression node. To do so, implement the function visitParenthesizedExpression in the Interpreter class, that should evaluate to the value of the expression.

Step 2: Records

A record is a collection of fields. Each field has a name and a value, which can be of any type. The record type is a way to group several values together. See it as a data class in Kotlin, a struct in C, a case class in Scala, a record in Java, etc.

In Alpine, a record (e.g. #pair(x: 1, y: 2)) is uniquely identified by:

its name (i.e. identifier, e.g. pair for the record #pair)
its arity (i.e. the number of fields it has, e.g. here #pair has 2)
the types of the fields it contains (e.g. here Int and Int)
its field labels (e.g. here x and y)

The interpreter should be able to create record instances when it visits a Record node. To do so, implement the function visitRecord in the Interpreter class, that should evaluate to a Record value.

Step 3: Conditional expressions

A conditional expression is an expression that evaluates to a value depending on a condition. In Alpine, a conditional expression is written as if <condition> then <then-branch> else <else-branch>. The interpreter should evaluate conditional expressions when it visits a Conditional node:

evaluate/visit the condition
if the condition evaluates to true, evaluate/visit the then-branch (successCase) and return its value
if the condition evaluates to false, evaluate/visit the else-branch (failureCase) and return its value

You can now implement the function visitConditional in the Interpreter class.

Step 4: Ascribed expressions

An ascribed expression is an expression that is annotated with a type. In Alpine, an ascribed expression is written as <expression> [@ | @! | @?] <type>. The interpreter should evaluate ascribed expressions when it visits an AscriptionExpression node.

There is multiple ways of changing the type of an expression:

Widening: when the type of the expression is a subtype of the ascribed type (). For example, 42 @ Int is valid because 42 is a subtype of Int. Moreover, 42 @ Any is valid because Int is a subtype of Any. This is called widening and is always safe and valid.
Unconditional narrowing: Narrowing is less safe. It is when the type of the expression is a supertype of the ascribed type. For example, 42 @! Float is invalid because Int is not a supertype of Float. let x: Int | Float = 42; x @! Int on the other hand is valid, because Int | Float is a supertype of Int. This is called narrowing and is not always safe and valid. In Alpine, narrowing can be done using the @! operator. If the ascribed type is not a subtype of the type of the expression, the interpreter should raise a Panic.
Safe narrowing: Instead of forcing such a narrowing, one can use the @? operator. It returns either a #none or a #some(T) depending on whether the narrowing is valid or not.

For example:

let x = 42 @ Any
let y = x @? Int

y will evaluate to #some(42). If the ascribed type is not a subtype of the type of the expression, the result is #none, e.g.

let x = 42 @ Any
let y = x @? Float

y will evaluate to #none.

Implement now the function visitAscribedExpression in the Interpreter class:

In case of a widening, return the value of the expression
In case of an unconditional narrowing, check if the ascribed type is a subtype of the type of the expression. If it is, return the value of the expression. Otherwise, raise a Panic.
In case of a safe narrowing, return #some(value) if the ascribed type is a subtype of the type of the expression, and #none otherwise.

Note that #none and #some are built-in records, and are defined in the Value object, in the evaluation/Value.scala file.

Step 5: Functions

A function is a block of code that can be called. In Alpine, a function is defined using the fun keyword. For example, the following code defines a function add that takes two arguments x and y and returns their sum:

fun add(_ x: Int, _ y: Int) -> Int {
  x + y
}

Note that by construction of our interpreter, it will never visit a Function node (since we are interpreting directly the entrypoint.)

However, we may encounter the definition of functions with a Let/Biding node:

let add = (_ x: Int, _ y: Int) -> Int {
  x + y
}

Such functions are called lambda functions (or anonymous functions). The interpreter should evaluate lambda functions when it visits a Let node. Moreover, these lambda functions can be defined inside a function in itself and has capture semantics. For example:

fun adder(_ x: Int, _ y: Int) -> Int {
  let add = (_ z: Int) -> Int {
    x + z // x is captured from the outer scope
  } {
    add(y)
  }
}
let main = print(adder(3, 4))

has the same behavior as the previous add function.

Here, the lambda captures the x variable from the outer scope. This is what the lambda "captured".

The _ in the function definitions are optional argument names. They can be used to make the code more readable by giving names to the arguments for interface/documentation purposes, that are are different from the variable names used in the body.

For instance:

fun scale(by f: Int) -> Int {
  10 * f
}
let main = print(scale(by: 2))

It lets the programer give more information about the argument to the caller, without having to use this name inside the function.

When the wildcard _ is used, you can not specify the name of the argument when calling the function. For instance, the following code is invalid:

fun scale(_ f: Int) -> Int {
  10 * f
}
let main = print(scale(f: 2))

This is the correct way to call the function:

fun scale(_ f: Int) -> Int {
  10 * f
}
let main = print(scale(2))

Step 5.1: Applications

An application is the act of calling a function. In Alpine, we differentiate applications in three categories:

Standard function call (Application): e.g. add(1, 2)
Binary operator call (InfixApplication): e.g. 1 + 2 which is equivalent to iadd(1, 2) (iadd is a built-in function)
Unary operator call (PrefixApplication): e.g. -1 which is equivalent to ineg(1) (ineg is a built-in function)

You should evaluate first:

the function i.e., retrieve the function node from the context using the identifier
the arguments i.e., evaluate/visit the arguments (call by value semantic)

Then, you should call the function on the arguments: you should use the call helper function.

Step 5.2: `call` function

Let's implement the call function.

The implementation for all the built-in functions is provided, you can take inspiration from it.

Your task is to implement the function for the following cases:

the function is not a built-in function and is not a lambda function
the function is not a built-in function but is a lambda function

In each case, the context needs to be updated with the new bindings.

In lambdas, do not forget about the captures!

Step 6: Let bindings

A let binding is how to define a new variable with a new block. We are already familiar with the top-level let binding (Binding node) but let's see the Let node.

In Alpine, a let binding is written as:

let x = 1 {
  x + 1
}

and here evaluates to 2.

In other words, a Let is a Binding with a body that is executed with the new binding in the context.

To do so, implement the function visitLet in the Interpreter class, that should:

evaluate/visit the definition of the Binding.
evaluate/visit the body of the Let with the new Binding.

Note that the Let has call-by value semantics (i.e. the definition of the Binding is evaluated before evaluating the body of the Let and every subsequent reference to that new variable makes reference to the value to which the definition was evaluated.)

Step 7: Pattern matching

Pattern matching is a way to match a value against a pattern. In Alpine, a pattern matching looks like:

match <scrutinee> {
  case <pattern> then <expression>
  // …
}

For instance,

match #person(age: 1) {
  case #person(age: 1) then 1
  case #person(age: 2) then 2
  case _ then -1
}

#person(age: 1) is the scrutinee and #person(age: 1), #person(age: 2) and _ are the patterns. Here, the expression evaluates to 1.

A pattern can be recursively constructed with the following elements:

A wildcard (_): matches any value
An exact value: matches the exact value
A record: matches a record with the same name and the same fields

Moreover, bindings can be defined in the patterns. For instance:

match someValue {
  case let p: #person(age: Int) then p.age >= 2
  case _ then false
}

In this case, the first expression of the first case is evaluated with a new binding: p is bound to the record #person of type #person(age: Int) (i.e., here, the scrutinee value).

For instance, if someValue was #person(age: 1), the first case would be executed and the expression would evaluate to false (because 1 >= 2 evaluates to false because p.age = 1.) If someValue was not a #person record, the second case would be executed and the expression would evaluate to false.

Similarly, you can have let inside records too:

match someValue {
  case #person(age: let n) then n >= 2
  case _ then false
}

and this has the same behavior as the previous example.

Similar to the Let node, the Match node has call-by value semantics (i.e., the scrutinee is first evaluated to a value before executing the match).

If the scrutinee does not match any pattern, the interpreter should raise a Panic.

If the scrutinee matches multiple patterns, the first pattern that matches is chosen.

Note as well that patterns can contain expressions. For instance, the following code is valid:

match #person(age: 2) {
  case #person(age: 1) then 1
  case #person(age: 1 + 1) then 2
  case _ then -1
}

and evaluates to 2.

So expressions in patterns should be evaluated before matching.

Step 7.1: Visiting the `Match` node

To implement the pattern matching, implement the helper function visitMatch in the Interpreter class, that should:

evaluate/visit the scrutinee
evaluate/visit the cases and return the value of the first case that matches the scrutinee

You can use the matches function to check if a pattern matches a value. matches returns either None if the pattern does not match the value, or Some(bindings) if the pattern matches the value. The bindings is a map from the variable identifiers to the values they are bound to, and of type Frame (that is a Map[symbols.Name, Value].)

These bindings should be used to evaluate the case return expression.

Step 6.2: Implementing `matches`

matches makes a call to either:

matchesWildcard
matchesValue
matchesRecord
matchesBinding

depending on the type of the pattern.

matchesWildcard matches any value and returns an empty Frame
matchesValue matches the exact value and returns an empty Frame
matchesBinding matches a binding and returns a Frame with the bindings if any. Be careful, the type of the binding must match as well.
matchesRecord matches a record with the same name and the same fields, and returns a Frame with the bindings if any. Note that the fields of the record can be expressions and that the patterns for the fields should be recursively matched. Be careful, the record type must match as well (you can use Pattern.record#dynamicType and structurallyMatches.)

Step 8: Lambdas

We have seen lambda functions. However, we have not implemented the visitLambda method.

Implement the function visitLambda in the Interpreter class, that should:

return a Lambda value
update the context with the new bindings and specify a new Frame for the captures.

As a reminder, the captures are the variables that are used in the lambda but are not defined in the lambda (i.e., whose values comes from the environment outside of the lambda). For instance, in the following code:

let x = 1
let add = (_ y: Int) -> Int {
  x + y
}

x is a capture of the add lambda.

Check the Context.flattenedmethod while implementing the captures.

Grammar

In this section, we give a list of built-in functions and a summary of the tree types you will work with.

Built-in functions

Here is a list of all the built-in functions, provided for reference:

equality(a: Any, b: Any) -> Bool: returns true if a and b are equal, false otherwise (a == b)
inequality(a: Any, b: Any) -> Bool: returns true if a and b are not equal, false otherwise (a != b)
lnot(a: Bool) -> Bool: returns true if a is false, false otherwise (!a)
land(a: Bool, b: Bool) -> Bool: returns true if a and b are true, false otherwise (a && b)
lor(a: Bool, b: Bool) -> Bool: returns true if a or b is true, false otherwise (a || b)
ineg(a: Int) -> Int: returns the negation of a (-a)
iadd(a: Int, b: Int) -> Int: returns the sum of a and b (a + b)
isub(a: Int, b: Int) -> Int: returns the difference of a and b (a - b)
imul(a: Int, b: Int) -> Int: returns the product of a and b (a * b)
idiv(a: Int, b: Int) -> Int: returns the division of a by b (a / b)
irem(a: Int, b: Int) -> Int: returns the remainder of a by b (a % b, commonly known as modulo.)
ishl(a: Int, b: Int) -> Int: returns the result of the left shift of a by b (a << b)
ishr(a: Int, b: Int) -> Int: returns the result of the right shift of a by b (a >> b)
ilt(a: Int, b: Int) -> Bool: returns true if a is strictly less than b, false otherwise
ile(a: Int, b: Int) -> Bool: returns true if a is less than or equal to b, false otherwise
igt(a: Int, b: Int) -> Bool: returns true if a is strictly greater than b, false otherwise
ige(a: Int, b: Int) -> Bool: returns true if a is greater than or equal to b, false otherwise
iinv(a: Int) -> Int: returns the bitwise inversion of a (~a)
iand(a: Int, b: Int) -> Int: returns the result of the bitwise and of a and b (a & b)
ior(a: Int, b: Int) -> Int: returns the result of the bitwise or of a and b (a | b)
ixor(a: Int, b: Int) -> Int: returns the result of the bitwise xor of a and b (a ^ b)
fneg(a: Float) -> Float: returns the negation of a (-a)
fadd(a: Float, b: Float) -> Float: returns the sum of a and b (a + b)
fsub(a: Float, b: Float) -> Float: returns the difference of a and b (a - b)
fmul(a: Float, b: Float) -> Float: returns the product of a and b (a * b)
fdiv(a: Float, b: Float) -> Float: returns the division of a by b (a / b)
flt(a: Float, b: Float) -> Bool: returns true if a is strictly less than b, false otherwise (a < b)
fle(a: Float, b: Float) -> Bool: returns true if a is less than or equal to b, false otherwise (a <= b)
fgt(a: Float, b: Float) -> Bool: returns true if a is strictly greater than b, false otherwise (a > b)
fge(a: Float, b: Float) -> Bool: returns true if a is greater than or equal to b, false otherwise (a >= b)

Tree types and summary

Here is a summary of the tree types you will work with, defined in the ast package.

ParenthesizedExpression(inner: Expression, …): (<expression>), evaluates to <expression>.
- The inner expression is inner.
- Example: ((1 + 2)) → ((3)) → (3) → 3
Record(identifier: String, fields: List[Labeled[Expression]], …):
- <identifier>, evaluates to a singleton record with the given name. (fields = Nil)
- <identifier>(<field1>: <expression1>, …), evaluates to a record with the given fields and values.
- <identifier> starts with #.
- Example: #pair(x: 1, y: 2), #singleton, #none
Conditional(condition: Expression, successCase: Expression, failureCase: Expression, …):
- if <condition> then <successCase> else <failureCase>, evaluates to <successCase> if <condition> evaluates to true, <failureCase> otherwise.
- Example: if true then 1 else 2 → 1, if false then 1 else 2 → 2
AscribedExpression(inner: Expression, operation: Typecast, ascription: Type, …):
- inner is the expression to be typecasted.
- @ means operation = Typecast.Widen
  - Returns the value of the expression. The check is left to the type-checker that is provided.
  - Examples: 1 @ Int, 1 @ Any
- @! means operation = Typecast.NarrowUnconditionally
  - Returns the value of the expression if the ascribed type is a subtype of the type of the expression. Otherwise, raises a Panic.
  - Examples: 1 @! Float
- @? means operation = Typecast.Narrow
  - Returns #some(value) if the ascribed type is a subtype of the type of the expression, and #none otherwise.
  - Examples: 1 @? Int, 1 @? Float
Application(function: Expression, arguments: List[Labeled[Expression]], …): <function>(<arguments>)
- function is the function to be called.
- arguments is the list of arguments.
- Example: iadd(1, 2)
PrefixApplication(function: Expression, argument: Expression, …): <operator> <expression>
- function is the function to be called.
- argument is the argument.
- Example: -1
- The function is resolved when parsing from <operator>.
InfixApplication(function: Expression, lhs: Expression, rhs: Expression): <lhs> <operator> <rhs>
- function is the function to be called.
- lhs and rhs are the arguments.
- Example: 1 + 2
- The function is resolved when parsing from <operator>.
Binding(identifier: String, ascription: Option[Type], initializer: Option[Expression], …):
- identifier is the name of the variable.
- ascription is the type of the variable, if given.
- initializer is the initial value of the variable, if given.
- Possible syntaxes:
  - let <identifier> = <expression>
  - let <identifier>: <ascription>
  - let <identifier>: <ascription> = <expression>
Let(binding: Binding, body: Expression, …): <binding> { <expression> }
- binding is the binding to be defined.
- body is the body of the let.
- Example: let x = 1 { x + 1 }
Match(scrutinee: Expression, cases: List[Match.Case], …): match <scrutinee> { <cases> }
- scrutinee is the value to be matched.
- cases is the list of cases.
- Example: match #person(age: 1) { case #person(age: 1) then 1 case _ then -1 }
Lambda(inputs: List[Parameter], output: Option[Type], body: Expression; …): (<parameters>) -> <output> { <body> }
- inputs is the list of input parameters.
- output is the output type, if given.
- body is the body of the lambda.
- Example: (_ x: Int, _ y: Int) -> Int { x + y }

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Readme.md

Readme.md

Lab 01 - Interpreter

Preparing the environment

Common issues

No autocompletion on Visual Studio Code (VSC)

Obtaining the lab files

Submit your work

General idea of the project

Lab formalities

Alpine language

The AST

Implementing the interpreter

Using SBT

Step 0: Entrypoint (provided)

Step 1: Parenthesized expressions

Step 2: Records

Step 3: Conditional expressions

Step 4: Ascribed expressions

Step 5: Functions

Step 5.1: Applications

Step 5.2: `call` function

Step 6: Let bindings

Step 7: Pattern matching

Step 7.1: Visiting the `Match` node

Step 6.2: Implementing `matches`

Step 8: Lambdas

Grammar

Built-in functions

Tree types and summary

Files

Readme.md

Latest commit

History

Readme.md

File metadata and controls

Lab 01 - Interpreter

Preparing the environment

Common issues

No autocompletion on Visual Studio Code (VSC)

Obtaining the lab files

Submit your work

General idea of the project

Lab formalities

Alpine language

The AST

Implementing the interpreter

Using SBT

Step 0: Entrypoint (provided)

Step 1: Parenthesized expressions

Step 2: Records

Step 3: Conditional expressions

Step 4: Ascribed expressions

Step 5: Functions

Step 5.1: Applications

Step 5.2: call function

Step 6: Let bindings

Step 7: Pattern matching

Step 7.1: Visiting the Match node

Step 6.2: Implementing matches

Step 8: Lambdas

Grammar

Built-in functions

Tree types and summary

Step 5.2: `call` function

Step 7.1: Visiting the `Match` node

Step 6.2: Implementing `matches`