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  • Rust

Idiom #295 String to Enum

Given the enumerated type T, create a function TryStrToEnum that takes a string s as input and converts it into an enum value of type T.

Explain whether the conversion is case sensitive or not.
Explain what happens if the conversion fails.

use std::str::FromStr;
enum MyEnum {
    Foo,
    Bar,
    Baz,
}

impl FromStr for MyEnum{
    type Err =  String;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        match s.to_lowercase().as_str() {
            "foo" => Ok(MyEnum::Foo),
            "bar" => Ok(MyEnum::Bar),
            "baz" => Ok(MyEnum::Baz),
            _ => Err("Could not convert str to enum".to_string())
        }
    }
}

Err could be an anyhow error or a custom struct
Alternatively use EnumString macro from strum crate.
X : T := T'Value (S);

Case-insensitive conversion of trimmed string S to enumeration type T. Constraint_Error is raised otherwise.
New implementation...
< > ⌨
Bart 
enum MyEnum {
    Foo,
    Bar,
    Baz,
}

impl FromStr for MyEnum{
    type Err =  String;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        match s.to_lowercase().as_str() {
            "foo" => Ok(MyEnum::Foo),
            "bar" => Ok(MyEnum::Bar),
            "baz" => Ok(MyEnum::Baz),
            _ => Err("Could not convert str to enum".to_string())
        }
    }
}

X : T := T'Value (S);
using namespace std::literals;

enum T {
  case_0,
  case_1,
  case_2,
  case_4 = 4
};

std::optional<T> TryStrToEnum(std::string_view s) {
  if (str == "case_0"sv) return case_0;
  if (str == "case_1"sv) return case_1;
  if (str == "case_2"sv) return case_2;
  if (str == "case_4"sv) return case_4;
  return std::nullopt;
}
static class StringToEnum {
  static bool TryStrToEnum<T>(
    string s, [NotNullWhen(true)] out T? eOut) where T : Enum {
    bool success = Enum.TryParse(typeof(T).GetType(), s, out var o);
    eOut = (T?)o;
    return success;
  }
}
T.values.byName(s);
def try_str_to_enum(t, string), do: t[string]
T x = T.valueOf(s);
function TryEnumToStr(const s: string; out enum: T): Boolean;
var
  code: integer;
begin
  Val(s, enum, code);
  Result := (code = 0);
end;
my %T = ( RED => 1, GREEN => 2, BLUE => 3 );

sub TryStrToEnum { my $s = shift; $T{uc $s} }

print GREEN;  # prints 2
print TryStrToEnum('BLUE');  prints 3


use constant { RED => 1, GREEN => 2, BLUE => 3 };

sub TryStrToEnum { eval uc shift }

say GREEN; # prints 2
say TryStrToEnum('BLUE'); # prints 3
t = T[s]