Assignment 5: Interpreter for a functional language

Implementing Programming Languages, 2012
Aarne Ranta (aarne (at)


The objective of this assignment is to write an interpreter for a small, untyped functional programming language. The interpreter should walk through programs and print out the value of the main function.

Before the work can be submitted, the interpreter has to pass some tests, which are given on the course web page via links later in this document.

The recommended implementation is via a BNF grammar processed by the BNF Converter (BNFC) tool. No type checker is needed.

The approximate size of the grammar is 15 rules, and the interpreter code should be 100 lines, depending on the programming language used.

All BNFC supported languages can be used, but guidance is guaranteed only for Haskell and Java.


Use a Makefile similar to Assignment 5. The interpreter should be compilable via calling


Calling the interpreter should work by the command

    ifun (-n|-v) <File>

The flag -n forces call-by-name evaluation, the flag -v forces call-by-value. The default, i.e. when no flag is present, is call-by-value.

Language specification

The language is the same as in the book, Chapter 7.

The main category is Program. A program is a sequence of definitions, which are terminated by semicolons. A definition is a function name followed by a (possibly empty) list of variable names followed by the equality sign = followed by an expression:

    fun x1 ... xn = exp

Both fun and the variables x1 ... xn are lexically identifiers. Thus fun is the function to be defined, and x1 ... xn are its arguments. These variables are considered bound in exp. Notice that the all such definitions can be converted to definitions of just fun with a lambda abstraction over its arguments.

Expressions are of the following forms

precedence expression example
3 identifier foo
3 integer 512
2 application f x
1 operation 3 + x
0 conditional if c then a else b
0 abstraction \x -> x + 1

Applications and operations are left-associative. Abstractions are right-associative.

The available operations are +, -, <.

Here is an example of a program in the language:

    -- example
    mult x y = 
      if (y < 1) then 0 else if (y < 2) then x else (x + (mult x (y-1))) ; 
    fact = \x -> if (x < 3) then x else mult x (fact (x-1)) ;
    main = fact 6 ;

Comments are line tails starting with --.


There is just one type of basic values: integers. Closures or abstraction expressions are also possible values of expressions.

Evaluation is parametrized so that it can be performed in both call-by-value and call-by-name manners.

The function defined in a definition is in scope in the entire program, including the expression part of that definition (which results in recursive and mutually recursive functions).

The variables bound on the left-hand-side of a definition are in scope in the expression part of the definition.

The variable x in an abstraction \x -> exp is bound in the body of the abstraction, i.e. exp.

Bindings made inside a scope overshadow those made outside.

The operations +, -, < have their usual integer semantics. The comparison < has value 1 if it is true, 0 if false.

The conditional if c then a else b is evaluated "lazily" so that if c has value 0, b is evaluated, otherwise a is evaluated.

The output of a program is the value of the main function, and it must be an integer.

A program may also exit with an error, due to an unbound identifier. It should then say what identifier is unbound. It is also an error if the main function is missing or has wrong type. Arithmetic operations on non-integers are also errors, e.g.

    f x = x + x ;
    main = f + f ;

All these errors occur at run time, because there is no type checker.

Example of success and failure

Source file

    -- file
    mult x y = 
      if (y < 1) then 0 else if (y < 2) then x else (x + (mult x (y-1))) ; 
    fact = \x -> if (x < 3) then x else mult x (fact (x-1)) ;
    main = fact 6 ;

Running the interpreter


Source file

    -- file
    mult x y = 
      if (y < 1) then 0 else if (y < 2) then x else (x + (mult x (y-1))) ; 
    fact = \x -> if (x < 3) then x else mul x (fact (x-1)) ;
    main = fact 6 ;

Running the interpreter

    ERROR: unknown identifier mul

Source file

    -- file
    grow x = 1 + grow x ;
    first x y = x ;
    main = first 5 (grow 4) ;

Running the interpreter

    <infinite loop>
    ./ifun -n

Compiling the interpreter

The interpreter is submitted as a source file package that can be compiled by typing make.

Test programs

Run the programs in the test suite before submitting your work. Include a log on the test run, showing the call of ifun for every program in the testsuite.

Success criteria

The interpreter must give acceptable results for the test suite and meet the specification in this document in all respects.

All "good" programs must work with at least one of the evaluation strategies; need not work on both (because of loop or long time); see comments in test programs to see which one is expected to work.

The solution must be written in an easily readable and maintainable way.