The objective of this assignment is to write an interpreter for a simple C-like programming language. The interpreter should run programs and correctly perform all their output actions.
Before the work can be submitted, the interpreter has to pass some tests, given in the example directory.
The recommended implementation is via a BNF grammar processed by the BNF Converter (BNFC) tool. The grammar built in Assignment 1 can be used.
Syntax-directed translation implemented with pattern matching.
You can take the code for MCalc as starting point.
The language is the same as in Assignment 1, and you can use the same grammar file.
There are five types of values:
true and false
Instead of boolean values, you may use integers.
Then true can be interpreted as 1 and false as 0.
Values can be seen as a special case of expressions: as expressions that contain no variables and cannot be evaluated further. But it is recommended to have a separate datatype of values, in order to guarantee that evaluation always results in a value.
Thus the evaluation of an expression in an environment should always result in a value.
The statements in a program are executed in
the order defined by their textual order as altered by while
loops and if conditions.
A declaration, e.g.
int i ;
adds a variable to the current context. Its value is initialized if and only if the declaration includes an initializing expression, e.g.
int i = 9 ;
An expression statement, e.g.
i++ ;
is evaluated, and its value is ignored.
A block of statements, e.g.
{
int i = 3 ;
i++ ;
}
is interpreted in an environment where a new context is pushed on the context stack at entrance, and popped at exit.
A while statement, e.g.
while (1 < 10){
i++ ;
}
is interpreted so that the condition expression is first evaluated.
If the value is true, the body is interpreted in the resulting
environment, and the while statement is executed again.
If the value is false, the statement after the while statement
is interpreted.
An if-else statement, e.g.
if (1 < 10) i++ ; else j++ ;
is interpreted so that the condition expression is first evaluated.
If the value is true, the statement before else is interpreted.
If the value is false, the statement after else is interpreted.
A print statement evaluates the argument expression and outputs
its value.
The interpretation of an expression, also called evaluation, returns a value whose type is determined by the type of the expression.
A literal, e.g.
123
3.14
true
"hello world"
is not evaluated further but just converted to the corresponding value.
A variable, e.g.
x
is evaluated by looking up its value in the innermost context where it occurs. If the variable is not in the context, or has no value there, the interpreter terminates with an error message
uninitialized variable x
A postincrement,
i++
has the same value as its body initially has
(here i). The value of the variable
i is then incremented by 1.
i-- correspondingly decrements i by 1.
If i is of type double, 1.0 is used instead.
A preincrement,
++i
has the same value as i plus 1. This incremented value
replaces the old value of i. The decrement and double
variants are analogous.
The arithmetic operations addition, subtraction, multiplication, and division,
a + b
a - b
a * b
a / b
are interpreted by evaluating their operands from left to right.
The resulting values are then added, subtracted, etc., by using
appropriate operations of the implementation language. We are not
picky about the precision chosen, but suggest for simplicity
that int should be int and double should be double.
Addition expressions for string arguments are interpreted by concatenation, without any intervening spaces.
Comparisons,
a < b
a > b
a >= b
a <= b
a == b
a != b
are treated similarly to the arithmetic operations, using comparisons of the implementation language. The returned value must be boolean (or an integer, if you use integers to represent booleans).
Conjunction,
a && b
is evaluated lazily: first a is evaluated. If the
result is true, also b is evaluated, and the
value of b is returned. However, if
a evaluates to false, then false
is returned without evaluating b.
Disjunction,
a || b
is also evaluated lazily: first a is evaluated. If the
result is false, also b is evaluated, and the
value of b is returned. However, if
a evaluates to true, then true
is returned without evaluating b.
Assignment,
x = a
is evaluated by first evaluating a. The resulting
value is returned, but also the context is changed by assigning
this value to the innermost occurrence of x.
The interpreter must be a program called imini, which is
executed by the command
imini <SourceFile>
and prints its output to the standard output. The output at success must be just the output defined by the interpreter.
The output at failure is an interpreter error, or a
SYNTAX ERROR as in Assignment 1.
Your interpreter must interpret all example programs.
Your solution must be written in an easily readable and maintainable way.
An easy way to test interpreter is to run the test file test2.sh.
In a Unix shell, you can use the command source test2.sh.
This assumes that
imini.hs is in the same directory
examples/ or a symbolic link to it in the same directory.
Running the test produces the file log2.txt, which you must submit
together with the file Interpreter.hs.
This is based on the extension of the grammar in Assignment 1 with functions and function calls, and uses the grammar that you wrote there.
The recommended procedure is two passes:
main()
The evaluation of a function call starts by evaluting the arguments and building an environment where the received values are assigned to the argument variables (a.k.a. parameters) of the function.
The statements in the body are then executed in
the order defined by their textual order as altered by while
loops and if conditions.
The function returns a value, which is obtained from a return
statement. This statement can be assumed to be the in the last statement of
the function body: either alone, or in the branches of an if-else
statement.
If the return type is void, no return statement is
required.
Your interpreter must interpret all additional example programs.