WHILE-Programme

while

Parser and interpreter for the while programming language

The while programming language is a verry simplified language, which is turing complete!

syntax specification via context-free type-2 grammar

The set of WHILE programs is the language generated by the following grammar

$$ G = (N, ∑, P, S) $$

where

$N = \{ Prog, Var, Const, Expr \}$

(the set of non terminal Symbols)

$∑ = \{ loop, while, do, end, +, -, ;, :=, != \} ∪ \mathbb{N}_0 ∪ VariableNames$

(the set of terminal symbols)

$S = Prog$

(the start non terminal)

$P = \{$

$Prog → // comment …$

$Prog → Prog; Prog$

$Prog → Var := Expr$

$Expr → Var \mid Const \mid Expr + Expr \mid Expr - Expr$

$Prog → \textrm{loop Expr do Prog end}$

$Prog → \textrm{while Expr != Expr do Prog end}$

$Var → VariableName$ (that has to start with ascii letter and can contain digits or underscore)

$Const → \mathbb{N}_0$

$\}$

(the set of production rules)

semantic

as expected, i guess … lol

NOTE: the subtraction is only defined in $\mathbb{N}_0$ therefore $5 - 8$ will result in $0$

also i added a max recursion depth like python3 to prevent users to write non terminating code … (the max rec depth for a while loop is 1500 iterations)

usage

example

lets say you have a function $ψ$ for calculating a simple multiplication $a ⋅ b$

$$ψ \colon \mathbb{N}^2_0 \longrightarrow \mathbb{N}_0$$

$$(x_1, x_2) \mapsto x_1 ⋅ x_2$$

lets calculate $420 ⋅ 69$ via $ψ(v)$ where $v := (420, 69)$, the result should be $28980$.

cat ./examples/multiplication-func.while

// init var state
rv := 0;
a := 420;
b := 69;

// loop will be exec b-times
while b != 0 do

    // calculate rv += a (b-times)
    //  -> a * b = a + a + ... b-times

    temp := rv;
    loop a do temp := temp + 1 end;
    rv := temp;

    b := b - 1
end

since this is a bit tidies i allowed users to use expressions in the context

• conditions
• additions
• and subtractions

(refer to the grammar defined above)

shortened (not commpletely while-language correct …) version

x0 := 0;
x1 := 420;
x2 := 69;

loop x2 do x0 := x0 + x1 end

how to call `stack run`

stack run

:

= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
�[91m[Error]�[0m: Usage: stack run <filename>
Use 'stack run <filename>' to interpret a while source file.
= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =

:

stack run ./examples/multiplication-func.while

= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =

�[92m[Interpreting file]�[0m    ./examples/multiplication-func.while
�[92m[READING .WHILE FILE]�[0m "./examples/multiplication-func.while"
�[92m[DONE TOKENIZE]�[0m
�[92m[DONE PARSING AST]�[0m
�[92m[RESULT OF EVALUATION]�[0m
[("temp",28980),("b",0),("a",420),("rv",28980)]

= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =

compile

stack build

run

stack run

test

stack test

linting

stack exec hlint src/*.hs app/*.hs test/*.hs

requirements

using `apt`

Haskell Tool Stack

curl -SL https://get.haskellstack.org/ | sh

Glasgow Haskell Compiler

apt install ghc

HLint

apt install hlint

using `nix`

nix-shell

hlint

hlint src/*.hs app/*.hs test/*.hs

build and run

same as before

LICENSE (BSD-3-Clause)

cat ./LICENSE

Copyright Felix Drees (c) 2023

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