Input and Parsing

Gruel can read input from the user with @read_line and convert strings to numbers with the @parse_* intrinsics.

Reading a Line

@read_line() reads one line from standard input and returns it as a String, with the newline stripped:

fn main() -> i32 {
    @dbg("What is your name?");
    let name = @read_line();
    @dbg("Hello,");
    @dbg(name);
    0
}

Running it:

$ ./program
What is your name?
Alice
Hello,
Alice

If EOF is reached with no input, the program panics with "unexpected end of input". A partial final line (no trailing newline) is returned as-is.

Parsing Integers

Use @parse_i32, @parse_i64, @parse_u32, or @parse_u64 to convert a string to a number. These borrow the string so it remains usable after the call:

fn main() -> i32 {
    @dbg("Enter a number:");
    let input = @read_line();
    let n = @parse_i32(input);
    @dbg(n * 2);
    0
}

Running it:

$ ./program
Enter a number:
21
42

If the string contains anything other than digits (and an optional leading - for signed types), the program panics with a clear error message.

All Parsing Intrinsics

The input must be exactly decimal digits with an optional leading - (signed only). No whitespace, no underscores, no prefixes like 0x.

A Complete Example

Here's a program that reads two numbers and prints their sum:

fn main() -> i32 {
    @dbg("First number:");
    let a = @parse_i32(@read_line());

    @dbg("Second number:");
    let b = @parse_i32(@read_line());

    let sum = a + b;
    @dbg(sum);

    0
}

Running it:

$ ./program
First number:
10
Second number:
32
42

Random Numbers

For programs that need random values, use @random_u32() or @random_u64():

fn main() -> i32 {
    let r = @random_u32();
    @dbg(r);  // prints a random 32-bit number
    0
}

These read from the platform's entropy source (getrandom on Linux, getentropy on macOS), so they're suitable for random number generation but not for cryptographic use.

Building an Interactive Program

Combining input, parsing, and control flow:

fn main() -> i32 {
    @dbg("Guess the number (between 1 and 10):");
    let secret = 7;
    let guess = @parse_i32(@read_line());

    if guess == secret {
        @dbg("Correct!");
    } else {
        @dbg("Wrong!");
        @dbg(secret);
    }

    0
}