Simple Fortran interpreter - Functional programming 2020/2021

This project implements some of the constructs from historical FORTRAN, written purely in Haskell.
Parser combinators inspired by this paper.

Compilation and usage

$ git clone https://github.com/hjaremko/fi.git && cd fi
$ ghc --make -o fi src/Main.hs -isrc
$ ./fi <filename>

Supported statements

Assumptions

• Every variable is typed float.
• Variables are initialized on the first use with zero.
• Whitespaces should be ignored.

Output

WRITE "Hello world! ", (2*4 + 2)

It is possible to print multiple expressions separeted by comma.

Input

READ A

Reads a float value from standard input.

Labels

Statements can be labeled with integer values. Labels should be unique.

Arithmetic IFs

IF (102 .EQ. 120) 40, 50, 60
40   WRITE "Negative"
50   WRITE "False"
60   WRITE "True"

Three-way arithmetic conditional statement. Jumps to three different branches depending on whether the result of an expression is negative, zero, or positive.

DO loops

DO (label) I=(init value), (stop value) [, (step value)]
    WRITE I
(label) CONTINUE

Simple loop, correct labels are required. Step value can be omitted (default is 1).

Assignments

I = 123
I = I * (200 + 10)

Jumps

GOTO 13
12 WRITE "This will be omitted"
13 WRITE "Hello world"

Arithmetic and logic expressions

Supported arithmetic operators: +, -, *, /, unary -, SQRT and (, ).
Supported logic operators: .EQ. (equals), .NE. (not equals), .LT. (less than), .LE. (less equals), .GT. (greater than), .GE. (greater equals).

True is indicated by value 1.0 and false by 0.0.

Example code

    WRITE "Square root calculator"
    DO 300 N=0,2
        WRITE "Calculation no. ", N
        READ A
        READ B
        READ C
        D=B*B-4*A*C
        WRITE "Delta: ", D
        IF (D) 101,102,103
        101  WRITE "No roots"
        GOTO 200
    102 X=2
        WRITE "Root: ",X
        GOTO 200
    103 X1=(-B-SQRT(D))/(2*A)
        X2=(-B+SQRT(D))/(2*A)
        WRITE "Root no. 1: ",X1
        WRITE "Root no. 2: ",X2
    200 WRITE "End of calculation no. ", N
    300 CONTINUE
    DO 600 N=0, 30, 10
        WRITE "N: ", N
        DO 700 K=0, 2
            WRITE "Hello from inner loop"
        700 CONTINUE
    600 CONTINUE
    WRITE "Hello World!"
    IF (102.EQ.120) 40, 50, 60
    40   WRITE "Negative"
    50   WRITE "False"
    60   WRITE "True"
    WRITE 10-1 .EQ. -9
    WRITE (1 + 1)+1.EQ.3
    Expr = 1 + 5 * 2 .EQ. 12 - 1
    WRITE "Expr: ", Expr
    WRITE "-50:  ", -50
    WRITE Expr, " <= -50:  ", Expr .LE. -50
    WRITE Expr, " >  -50:  ", Expr .GT. -50
    IF (Expr  .LE. -50) 11, 11, 12
    11 WRITE "False"
    GOTO 13
    12 WRITE "True"
    13 WRITE "End"