Which programming languages don't have runtime exceptions?

Question

string name = null; name.ToLower();

In most languages such code would compile. Which languages would catch this and the like errors at compile time ?

The only one I know uptil now is elm: http://elm-lang.org

Answer 1

Rust prevents a lot of errors at compile-time. Rust doesn't have runtime exceptions (other than panic! which crashes the program), instead it uses return values for error handling.

let name = None; // error: mismatched types
let name: Option<String> = None; // ok

name.to_lowercase(); // error: no method named `to_lowercase` found for type `Option`

// correct:
match name {
    None => { /* handle None */ },
    Some(value) => { value.to_lowercase(); },
}

// or to ignore None and crash if name == None
name.unwrap().to_lowercase();

One of Rust's core concepts that no other language has (afaik) is lifetimes, which prevent dangling references at compile-time. However this is something that garbage-collected and reference-counted languages don't need.

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Go doesn't have exceptions. Like Rust, it uses return values to signal errors. But it's not null-safe:

// strings can't be nil:
var name string = nil // error: cannot use nil as type string in assignment

// string pointers can:
var name *string = nil // ok

string.ToLower(*name) // panic: runtime error: invalid memory address

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The following languages do have exception handling but might still be helpful to answer the question.

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Swift is also safer than average, but it does include runtime exceptions. However, Swift's exceptions are more explicit than those of C++, Java, C#, etc. (e.g. you have to prefix each call to a throwing function with try, and a function declaration must specify whether that function may throw).

let name = nil // error: type of expression is ambiguous without more context
let name: String? = nil // ok

name.lowercaseString // error: value of optional type 'String?' not unwrapped; did you mean to use '!' or '?'?
name!.lowercaseString // will crash at runtime with: fatal error: unexpectedly found nil while unwrapping an Optional value
name?.lowercaseString // calls method only if name != nil

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Kotlin is a safer JVM-language which also compiles to JavaScript. Kotlin also has exceptions.

val name = null
name.toLowerCase() // compile-error

if (name != null)
    name.toLowerCase() // ok

name?.toLowerCase() // call only if non-null

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Ada is a language designed for safety-critical purposes. According to http://www.adaic.org/advantages/features-benefits/, its "many built-in checks allow the compiler or linker to detect errors that in a C-based language would only be caught during run-time".

Answer 2

Elm, as the OP noted, is an excellent language without runtime exceptions. Another language that seems to be equally committed to safety is Pony.

• https://www.ponylang.io/discover/#exception-safe

Answer 3

Here's a list of languages that by default don't allow variables to be null.

This makes them a lot safer, preventing all NullPointerExceptions at compile-time, and clearer because you know where you're allowed to pass null and where not, simply by looking at the code.

Most of these languages also provide many other compile-time safety features.