What causes "This is an extra type specifier" in gleam-lang/gleam?

Remove the redundant type specifier, keeping only one type per segment

This is an extra type specifier

Error message

This is an extra type specifier

What it means

Raised as ErrorType::ConflictingTypeOptions when a bit array segment specifies more than one type option (e.g. int, float, bytes, bits, utf8, utf16, utf32, or utf*_codepoint). A segment can have exactly one type; the second type option triggers this error at the location of the offending option.

Solutions

Remove the redundant type specifier, keeping only one type per segment
If you intended a sizing option, use 'size:n' or 'unit:n' instead of a second type
Merge intent into a single type that matches the data (e.g. choose bytes vs utf8, not both)

Example fix

// before
let assert <<header:32:float>> = data
// after
let assert <<header:float>> = data

Defensive patterns

Strategy: validation

Validate before calling

// Count type options before writing a segment: exactly one of
// int | float | bytes | bits | utf8 | utf16 | utf32 | utf8_codepoint | utf16_codepoint | utf32_codepoint
fn count_type_options(spec: String) -> Int {
  ["int", "float", "bytes", "bits", "utf8", "utf16", "utf32"]
  |> list.filter(fn(t) { string.contains(spec, ":" <> t) })
  |> list.length
}

Prevention

One type option per segment — always verify after edits
Remember size/unit are modifiers, not types; use them instead of adding a second type
Let the compiler/linter read the segment aloud: type, then size, then unit, then signedness, then endianness

When it happens

Trigger: Writing two type specifiers on one segment, such as <<x:int:float>> or <<data:bytes:utf8>>, in either a bit array expression or a pattern.

Common situations: Typos where the author meant a size or unit option but wrote a second type; copy-paste edits that left a stale type option behind; misunderstanding that ':utf8' is a type, not a modifier.

Related errors

AI-assisted analysis of gleam-lang/gleam@15b07c7830 (2026-09-14). Data as JSON: /api/errors/94389fab5c18266a. Report an issue: GitHub.

Appendix: source

Thrown at compiler-core/src/bit_array.rs:135

                        target,
                        option: UnsupportedOption::UtfCodepointPattern,
                    },
                    option.location(),
                );
            }

            Bytes { .. }
            | Int { .. }
            | Float { .. }
            | Bits { .. }
            | Utf8 { .. }
            | Utf16 { .. }
            | Utf32 { .. }
            | Utf8Codepoint { .. }
            | Utf16Codepoint { .. }
            | Utf32Codepoint { .. } => {
                if let Some(previous) = categories.type_ {
                    return err(
                        ErrorType::ConflictingTypeOptions {
                            existing_type: previous.label(),
                        },
                        option.location(),
                    );
                } else {
                    categories.type_ = Some(option);
                }
            }

            Signed { .. } | Unsigned { .. } => {
                if let Some(previous) = categories.signed {
                    return err(
                        ErrorType::ConflictingSignednessOptions {
                            existing_signed: previous.label(),
                        },
                        option.location(),
                    );

View on GitHub (pinned to 15b07c7830)