Logo

Programming-Idioms

This language bar is your friend. Select your favorite languages!
  • Perl

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

sub TryStrToEnum { eval uc shift }

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

Perl is not a strictly typed language and has no builtin enum. The nearest equivalent is 'use constant', which can be used to define names that can be used like constants. Under the covers they are actually subroutines which return the associated value. There are also CPAN routines (like 'enum') which provide more flexibility. This implementation is case insensitive, courtesy of uc (uppercase). Conversion failure returns undef.
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

Perl is not a strictly typed language and has no builtin enum. One approach is to use a hash (similar to a Python map) -- but it won't catch a name misspelling at compile time error. See 'use constant' and CPAN's 'enum' for better options. This implementation is case insensitive, courtesy of uc (uppercase). Conversion failure returns undef.
X : T := T'Value (S);

Case-insensitive conversion of trimmed string S to enumeration type T. Constraint_Error is raised otherwise.
New implementation...
< > ⌨
Bart 
use constant { RED => 1, GREEN => 2, BLUE => 3 };

sub TryStrToEnum { eval uc shift }

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


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;
t = T[s]
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())
        }
    }
}