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IMP is a small language of while programs. IMP is called an "imperative" language because program execution involeves carrying out a series of explicit commands to change state.

The formal semantics of programming languages - Glynn Winskel

This OCaml project implements a simple interpreter of the IMP programming language described by Winskell. The language is extended with some tweaks which are not present in the book, such as the print and read functions. The interpreter makes use of ocamllex, menhir and ppx as dependencies. To build the executable run dune run and to execute type dune exec imp.

Be aware that this implementation is a toy, it surely has some not checked errors 🙂

The grammar is expressed using the BNF.

Arithmetic Expressions
a ::= n | X | a0 + a1 | a0 - a1 | a0 * a1 | read | ( a )

Boolean Expressions
b ::= true | false | a0 = a1 | a0 <= a1 | a0 < a1 | a0 >= a1 | a0 > a1 | !b | b0 || b1 | b0 && b1 | ( b )

Commands
c ::= skip | print a | c0; c1 | if b then c0 else c1 end | while b do c end

The program shown below implement the factorial function.

n := read;
f := 1;
while n > 0 do
    f := f * n;
    n := n - 1;
end
print f;

All the examples can be found inside the playground directory.

The features added to the language are a few:

• the end keyword has been introduced to mark the end of while/if block;
• the read expression is useful to read a number from the standard input (⚠️ No check is performed onto the typed input);
• the print command is the most complex one: it prints a number onto the screen.