[ufo]

Why define Format as

    data Format = FInt Format
                | FString Format
                | FOther Char Format
                | FEnd

instead of using lists?

     data FormatPart = FInt | FString | FOther Char
     type Format = List FormatPart

[walrus]

Seems to work fine: https://gist.github.com/anonymous/18e4d6823de55a4d0567