Type::Tiny::Manual::UsingWithMoo - phpMan

NAME
    Type::Tiny::Manual::UsingWithMoo - basic use of Type::Tiny with Moo

MANUAL
  Type Constraints
    Consider the following basic Moo class:

      package Horse {
        use Moo;
        use namespace::autoclean;

        has name       => ( is => 'ro' );
        has gender     => ( is => 'ro' );
        has age        => ( is => 'rw' );
        has children   => ( is => 'ro', default => sub { [] } );
      }

    Code like this seems simple enough:

      my $br = Horse->new(name => "Bold Ruler", gender => 'm', age => 16);
      push @{ $br->children },
        Horse->new(name => 'Secretariat', gender => 'm', age => 0);

    However, once you step away from very simple use of the class, things
    can start to go wrong. When we push a new horse onto "@{ $br->children
    }", we are assuming that "$br->children" returned an arrayref.

    What if the code that created the $br horse had instantiated it like
    this?

      my $br = Horse->new(name => "Bold Ruler", children => 'no');

    It is for this reason that it's useful for the Horse class to perform
    some basic sanity-checking on its own attributes.

      package Horse {
        use Moo;
        use Types::Standard qw( Str Num ArrayRef );
        use namespace::autoclean;

        has name       => ( is => 'ro', isa => Str );
        has gender     => ( is => 'ro', isa => Str );
        has age        => ( is => 'rw', isa => Num );
        has children   => (
          is      => 'ro',
          isa     => ArrayRef,
          default => sub { return  [] },
        );
      }

    Now, if you instantiate a horse like this, it will throw an error:

      my $br = Horse->new(name => "Bold Ruler", children => 'no');

    The first type constraint we used here was Str. This is type constraint
    that requires values to be strings.

    Note that although "undef" is not a string, the empty string is still a
    string and you will often want to check that a string is non-empty. We
    could have done this:

      use Types::Common::String qw( NonEmptyStr );
      has name => ( is => 'ro', isa => NonEmptyStr );

    While most of the type constraints we will use in this manual are
    defined in Types::Standard, the Types::Common::String type library also
    defines many useful type constraints.

    We have required the horse's age to be a number. This is also a common,
    useful type constraint. If we want to make sure it's a whole number, we
    could use:

      use Types::Standard qw( Int );
      has age => ( is => 'rw', isa => Int );

    Or because negative numbers make little sense as an age:

      use Types::Common::Numeric qw( PositiveOrZeroInt );
      has age => ( is => 'rw', isa => PositiveOrZeroInt );

    The Types::Common::Numeric library defines many useful subtypes of Int
    and Num, such as PositiveInt and PositiveOrZeroInt.

    The last type constraint we've used in this example is ArrayRef. This
    requires the value to be a reference to an array.

    Types::Standard also provides HashRef and CodeRef type constraints. An
    example of using the latter:

      package Task {
        use Moo;
        use Types::Standard qw( CodeRef Bool );
        has on_success => ( is => 'ro', isa => CodeRef );
        has on_failure => ( is => 'ro', isa => CodeRef );
        has finished   => ( is => 'ro', isa => Bool, default => 0 );
        ...;
      }

      my $task = Task->new(
        on_success => sub { ... },
        on_failure => sub { ... },
        ...,
      );

    The Bool type constraint accepts "1" as a true value, and "0", "", or
    undef as false values. No other values are accepted.

    There exists an Object type constraint that accepts any blessed object.

      package Horse {
        use Moo;
        use Types::Standard qw( Object );
        use namespace::autoclean;

        ...;  # name, gender, age, children
        has father => ( is => 'ro', isa => Object );
        has mother => ( is => 'ro', isa => Object );
      }

    Finally, another useful type constraint to know about is Any:

      use Types::Standard qw( Any );
      has stuff => ( is => 'rw', isa => Any );

    This type constraint allows any value; it is essentially the same as not
    doing any type check, but makes your intent clearer. Where possible,
    Type::Tiny will optimize away this type check, so it should have little
    (if any) impact on performance.

  Parameterized Types
    Let's imagine we want to keep track of our horse's race wins:

      package Horse {
        use Moo;
        use Types::Standard qw( Str Num ArrayRef );
        use namespace::autoclean;

        ...;  # name, gender, age, children
        has wins => (
          is      => 'ro',
          isa     => ArrayRef,
          default => sub { return [] },
        );
      }

    We can create a horse like this:

      my $br = Horse->new(
        name    => "Bold Ruler",
        gender  => 'm',
        age     => 4,
        wins    => ["Futurity Stakes 1956", "Juvenile Stakes 1956"],
      );

    The list of wins is an arrayref of strings. The ArrayRef type constraint
    prevents it from being set to a hashref, for example, but it doesn't
    ensure that everything in the arrayref is a string. To do that, we need
    to parameterize the type constraint:

      has wins => (
        is      => 'ro',
        isa     => ArrayRef[Str],
        default => sub { return [] },
      );

    Thanks to the ArrayRef[Str] parameterized type, the constructor will
    throw an error if the arrayref you pass to it contains anything
    non-string.

    An alternative way of writing this is:

      has wins => (
        is      => 'ro',
        isa     => ArrayRef->of(Str),
        default => sub { return [] },
      );

    Which way you choose is largely a style preference. TIMTOWTDI!

    Note that although the constructor and any setter/accessor method will
    perform type checks, it is possible to bypass them using:

      push @{ $br->wins }, $not_a_string;

    The constructor isn't being called here, and although the accessor *is*
    being called, it's being called as a reader, not a writer, so never gets
    an opportunity to inspect the value being added. (It is possible to use
    "tie" to solve this, but that will be covered later.)

    And of course, if you directly poke at the underlying hashref of the
    object, all bets are off:

      $br->{wins} = $not_an_arrayref;

    So type constraints do have limitations. Careful API design (and not
    circumventing the proper API) can help.

    The HashRef type constraint can also be parameterized:

      package Design {
        use Moo;
        use Types::Standard qw( HashRef Str );
        has colours => ( is => 'ro', isa => HashRef[Str] );
      }

      my $eiffel65 = Design->new(
        colours => { house => "blue", little_window => "blue" },
      );

    The HashRef[Str] type constraint ensures the *values* of the hashref are
    strings; it doesn't check the keys of the hashref because keys in Perl
    hashes are always strings!

    If you do need to constrain the keys, it is possible to use a
    parameterized Map constraint:

      use Types::Common::String qw( NonEmptyStr );
      use Types::Standard qw( Map );
      has colours => ( is => 'ro', isa => Map[NonEmptyStr, NonEmptyStr] );

    Map takes two parameters; the first is a type to check keys against and
    the second is a type to check values against.

    Another useful type constraint is the Tuple type constraint.

      use Types::Standard qw( ArrayRef Tuple );
      use Types::Common::Numeric qw( PositiveInt );
      use Types::Common::String qw( NonEmptyStr );

      has wins => (
        is      => 'ro',
        isa     => ArrayRef[ Tuple[PositiveInt, NonEmptyStr] ],
        default => sub { return [] },
      );

    The Tuple[PositiveInt, NonEmptyStr] type constraint checks that a value
    is a two-element arrayref where the first element is a positive integer
    and the second element is a non-empty string. For example:

      my $br = Horse->new(
        name    => "Bold Ruler",
        wins    => [
          [ 1956, "Futurity Stakes" ],
          [ 1956, "Juvenile Stakes" ],
        ],
      );

    As you can see, parameterized type constraints may be nested to
    arbitrary depth, though of course the more detailed your checks become,
    the slower they will perform.

    It is possible to have tuples with variable length. For example, we may
    wish to include the jockey name in our race wins when it is known.

      use Types::Standard qw( ArrayRef Tuple Optional );
      use Types::Common::Numeric qw( PositiveInt );
      use Types::Common::String qw( NonEmptyStr );

      has wins => (
        is      => 'ro',
        isa     => ArrayRef[
          Tuple[ PositiveInt, NonEmptyStr, Optional[NonEmptyStr] ]
        ],
        default => sub { return [] },
      );

    The third element will be checked if it is present, but forgiven if it
    is absent.

    Or we could just allow tuples to contain an arbitrary list of strings
    after the year and race name:

      use Types::Standard qw( ArrayRef Tuple Str slurpy );
      use Types::Common::Numeric qw( PositiveInt );
      use Types::Common::String qw( NonEmptyStr );

      has wins => (
        is      => 'ro',
        isa     => ArrayRef[
          Tuple[ PositiveInt, NonEmptyStr, slurpy ArrayRef[Str] ]
        ],
        default => sub { return [] },
      );

    The "slurpy" indicator will "slurp" all the remaining items in the tuple
    into an arrayref and check it against ArrayRef[Str].

    It's even possible to do this:

      use Types::Standard qw( ArrayRef Tuple Any slurpy );
      use Types::Common::Numeric qw( PositiveInt );
      use Types::Common::String qw( NonEmptyStr );

      has wins => (
        is      => 'ro',
        isa     => ArrayRef[
          Tuple[ PositiveInt, NonEmptyStr, slurpy Any ]
        ],
        default => sub { return [] },
      );

    With this type constraint, any elements after the first two will be
    slurped into an arrayref and we don't check that arrayref at all. (In
    fact, the implementation of the Tuple type is smart enough to not bother
    creating the temporary arrayref to check.)

    Dict is the equivalent of Tuple for checking values of hashrefs.

      use Types::Standard qw( ArrayRef Dict Optional );
      use Types::Common::Numeric qw( PositiveInt );
      use Types::Common::String qw( NonEmptyStr );

      has wins => (
        is      => 'ro',
        isa     => ArrayRef[
          Dict[
            year    => PositiveInt,
            race    => NonEmptyStr,
            jockey  => Optional[NonEmptyStr],
          ],
        ],
        default => sub { return [] },
      );

    An example of using it:

      my $br = Horse->new(
        name    => "Bold Ruler",
        wins    => [
          { year => 1956, race => "Futurity Stakes", jockey => "Eddie" },
          { year => 1956, race => "Juvenile Stakes" },
        ],
      );

    The slurpy indicator does work for Dict too:

      Dict[
        year    => PositiveInt,
        race    => NonEmptyStr,
        jockey  => Optional[NonEmptyStr],
        slurpy HashRef[Str],  # other Str values allowed
      ]

    And "slurpy Any" means what you probably think it means:

      Dict[
        year    => PositiveInt,
        race    => NonEmptyStr,
        jockey  => Optional[NonEmptyStr],
        slurpy Any,  # allow hashref to contain absolutely anything else
      ]

    Going back to our first example, there's an opportunity to refine our
    ArrayRef constraint:

      package Horse {
        use Moo;
        use Types::Standard qw( Str Num ArrayRef );
        use namespace::autoclean;

        has name       => ( is => 'ro', isa => Str );
        has gender     => ( is => 'ro', isa => Str );
        has age        => ( is => 'rw', isa => Num );
        has children   => (
          is      => 'ro',
          isa     => ArrayRef[ InstanceOf["Horse"] ],
          default => sub { return [] },
        );
      }

    The InstanceOf["Horse"] type constraint checks that a value is a blessed
    object in the Horse class. So the horse's children should be an arrayref
    of other Horse objects.

    Internally it just checks "$_->isa("Horse")" on each item in the
    arrayref.

    It is sometimes useful to instead check "$_->DOES($role)" or
    "$_->can($method)" on an object. For example:

      package MyAPI::Client {
        use Moo;
        use Types::Standard qw( HasMethods );

        has ua => (is => 'ro', isa => HasMethods["get", "post"] );
      }

    The ConsumerOf and HasMethods parameterizable types allow you to easily
    check roles and methods of objects.

    The Enum parameterizable type allows you to accept a more limited set of
    string values. For example:

      use Types::Standard qw( Enum );
      has gender => ( is => 'ro', isa => Enum["m","f"] );

    Or if you want a little more flexibility, you can use StrMatch which
    allows you to test strings against a regular expression:

      use Types::Standard qw( StrMatch );
      has gender => ( is => 'ro', isa => StrMatch[qr/^[MF]/i] );

    Or StrLength to check the maximum and minimum length of a string:

      use Types::Common::String qw( StrLength );
      has name => ( is => 'ro', isa => StrLength[3, 100] );

    The maximum can be omitted.

    Similarly, the maximum and minimum values for a numeric type can be
    expressed using IntRange and NumRange:

      use Types::Common::Numeric qw( IntRange );
      # values over 200 are probably an input error
      has age => ( is => 'ro', isa => IntRange[0, 200] );

    Parameterized type constraints are one of the most powerful features of
    Type::Tiny, allowing a small set of constraints to be combined in useful
    ways.

  Type Coercions
    It is often good practice to be liberal in what you accept.

      package Horse {
        use Moo;
        use Types::Standard qw( Str Num ArrayRef Bool );
        use namespace::autoclean;

        ...;  # name, gender, age, children, wins
        has is_alive => ( is => 'rw', isa => Bool, coerce => 1 );
      }

    The "coerce" option indicates that if a value is given which *does not*
    pass the Bool type constraint, then it should be coerced (converted)
    into something that does.

    The definition of Bool says that to convert a non-boolean to a bool, you
    just do "!! $non_bool". So all of the following will be living horses:

      Horse->new(is_alive => 42)
      Horse->new(is_alive => [])
      Horse->new(is_alive => "false") # in Perl, string "false" is true!

    Bool is the only type constraint in Types::Standard that has a coercion
    defined for it. The NumericCode, UpperCaseStr, LowerCaseStr,
    UpperCaseSimpleStr, and LowerCaseSimpleStr types from
    Types::Common::String also have conversions defined.

    The other built-in constraints do not define any coercions because it
    would be hard to agree on what it means to coerce from, say, a HashRef
    to an ArrayRef. Do we keep the keys? The values? Both?

    But it is pretty simple to add your own coercions!

      use Types::Standard qw( ArrayRef HashRef Str );
      has things => (
        is     => 'rw',
        isa    => ArrayRef->plus_coercions(
          HashRef,     sub { [ values %$_ ] },
          Str,         sub { [ split /;/, $_ ] },
        ),
        coerce => 1,
      );

    (Don't ever forget the "coerce => 1"!)

    If a hashref is provided, the values will be used, and if a string is
    provided, it will be split on the semicolon. Of course, if an arrayref
    if provided, it already passes the type constraint, so no conversion is
    necessary.

    The coercions should be pairs of "from types" and code to coerce the
    value. The code can be a coderef (as above) or just string of Perl code
    (as below). Strings of Perl code can usually be optimized better by
    Type::Tiny's internals, so are generally preferred. Thanks to Perl's
    "q{...}" operator, they can look just as clean and pretty as coderefs.

      use Types::Standard qw( ArrayRef HashRef Str );
      has things => (
        is     => 'rw',
        isa    => ArrayRef->plus_coercions(
          HashRef,     q{ values %$_ },
          Str,         q{ [ split /;/, $_ ] },
        ),
        coerce => 1,
      );

    Coercions are deeply applied automatically, so the following will do
    what you expect.

      has inputs => (
        is     => 'ro',
        isa    => ArrayRef->of(Bool),
        coerce => 1
      );

    I am, of course, assuming you expect something like:

      my $coerced = [ map { !!$_ } @$orig ];

    If you were assuming that, congratulations! We are on the same
    wavelength.

    And of course you can still add more coercions to the inherited ones...

      has inputs => (
        is     => 'ro',
        isa    => ArrayRef->of(Bool)->plus_coercions(Str, sub {...}),
        coerce => 1
      );

  Method Parameters
    So far we have just concentrated on the definition of object attributes,
    but type constraints are also useful to validate method parameters.

    Let's remember our attribute for keeping track of a horse's race wins:

      use Types::Standard qw( ArrayRef Tuple Optional );
      use Types::Common::Numeric qw( PositiveInt );
      use Types::Common::String qw( NonEmptyStr );

      has wins => (
        is      => 'ro',
        isa     => ArrayRef[
          Tuple[ PositiveInt, NonEmptyStr, Optional[NonEmptyStr] ]
        ],
        default => sub { return [] },
      );

    Because we don't trust outside code to push new entries onto this array,
    let's define a method in our class to do it.

      package Horse {
        ...;

        sub add_win {
          my $self = shift;
          my ($year, $race, $jockey) = @_;
          my $win = [
            $year,
            $race,
            $jockey ? $jockey : (),
          ];
          push @{ $self->wins }, $win;
          return $self;
        }
      }

    This works pretty well, but we're still not actually checking the values
    of $year, $race, and $jockey. Let's use Type::Params for that:

      package Horse {
        use Types::Common::Numeric qw( PositiveInt );
        use Types::Common::String qw( NonEmptyStr );
        use Type::Params qw( compile );
        ...;

        sub add_win {
          state $check = compile(
            PositiveInt,
            NonEmptyStr,
            NonEmptyStr, { optional => 1 },
          );

          my $self = shift;
          my ($year, $race, $jockey) = $check->(@_);
          my $win = [
            $year,
            $race,
            $jockey ? $jockey : (),
          ];
          push @{ $self->wins }, $win;
          return $self;
        }
      }

    The first time this method is called, it will compile a coderef called
    $check. Then every time it is run, $check will be called to check the
    method's parameters. It will throw an exception if they fail. $check
    will also perform coercions if types have them (and you don't even need
    to remember "coerce => 1"; it's always automatic) and can even add in
    defaults:

      state $check = compile(
        PositiveInt,
        NonEmptyStr,
        NonEmptyStr, { default => sub { "Eddie" } },
      );

    On older versions of Perl (prior to 5.10), "state" variables are not
    available. A workaround is to replace this:

      sub foo {
        state $x = bar();
        ...;
      }

    With this:

      {         # outer braces prevent other subs seeing $x
        my $x;  # declare $x before sub foo()
        sub foo {
          $x = bar();
          ...;
        }
      }

    (While we're having a general Perl syntax lesson, I'll note that &$check
    with an ampersand and no parentheses is a shortcut for "$check->(@_)"
    and actually runs slightly faster because it reuses the @_ array for the
    called coderef. A lot of people dislike calling subs with an ampersand,
    so we will stick to the "$check->(@_)" syntax in these examples. But do
    consider using the shortcut!)

    The generalized syntax for "compile" is:

      state $check = compile(
        \%general_options,
        TypeForFirstParam,  \%options_for_first_param,
        TypeForSecondParam, \%options_for_second_param,
        ...,
      );

    As a shortcut for the "{ optional => 1 }}" option, you can just use
    Optional like in Tuple.

      state $check = compile(
        PositiveInt,
        NonEmptyStr,
        Optional[NonEmptyStr],
      );

    You can also use 0 and 1 as shortcuts for Optional[Any] and Any. The
    following checks that the first parameter is a positive integer, the
    second parameter is required (but doesn't care what value it is) and the
    third parameter is allowed but not required.

      state $check = compile(PositiveInt, 1, 0);

    It is possible to accept a variable number of values using "slurpy":

      package Horse {
        use Types::Common::Numeric qw( PositiveInt );
        use Types::Common::String qw( NonEmptyStr );
        use Types::Standard qw( ArrayRef slurpy );
        use Type::Params qw( compile );
        ...;

        sub add_wins_for_year {
          state $check = compile(
            PositiveInt,
            slurpy ArrayRef[NonEmptyStr],
          );

          my $self = shift;
          my ($year, $races) = $check->(@_);
          for my $race (@$races) {
            push @{ $self->wins }, [$year, $win];
          }
          return $self;
        }
      }

    It would be called like this:

      $bold_ruler->add_wins_for_year(
        1956,
        "Futurity Stakes",
        "Juvenile Stakes",
      );

    The additional parameters are slurped into an arrayref and checked
    against ArrayRef[NonEmptyStr].

    Optional parameters are only allowed after required parameters, and
    slurpy parameters are only allowed at the end. (And there can only be a
    at most one slurpy parameter!)

    For methods that accept more than one or two parameters, it is often a
    good idea to provide them as a hash. For example:

      $horse->add_win(
        year    => 1956,
        race    => "Futurity Stakes",
        jockey  => "Eddie",
      );

    This can make your code more readable.

    To accept named parameters, use "compile_named" instead of "compile".

      package Horse {
        use Types::Common::Numeric qw( PositiveInt );
        use Types::Common::String qw( NonEmptyStr );
        use Type::Params qw( compile_named );
        ...;

        sub add_win {
          state $check = compile_named(
            year    => PositiveInt,
            race    => NonEmptyStr,
            jockey  => NonEmptyStr, { optional => 1 },
          );

          my $self = shift;
          my $args = $check->(@_);
          my $win = [
            $args->{year},
            $args->{race},
            exists($args->{jockey}) ? $args->{jockey} : (),
          ];
          push @{ $self->wins }, $win;
          return $self;
        }
      }

    "compile" and "compile_named" work pretty much the same, except the
    latter accepts named parameters instead of positional, and returns a
    hashref.

    It will automatically allow for a hashref to be provided instead of a
    full hash. The following both work, but the $args variable will always
    be given a hashref.

      $horse->add_win({
        year    => 1956,
        race    => "Juvenile Stakes",
      });
      $horse->add_win(
        year    => 1956,
        race    => "Futurity Stakes",
        jockey  => "Eddie",
      );

    Well... I say "always" but you can tell "compile_named" to accept named
    parameters but return a positional list of parameters:

      package Horse {
        use Types::Common::Numeric qw( PositiveInt );
        use Types::Common::String qw( NonEmptyStr );
        use Type::Params qw( compile_named );
        ...;

        sub add_win {
          state $check = compile_named(
            { named_to_list => 1 },
            year    => PositiveInt,
            race    => NonEmptyStr,
            jockey  => NonEmptyStr, { optional => 1 },
          );

          my $self = shift;
          my ($year, $race, $jockey) = $check->(@_);
          my $win = [
            $year,
            $race,
            $jockey ? $jockey : (),
          ];
          push @{ $self->wins }, $win;
          return $self;
        }
      }

    Optional and slurpy named parameters are supported as you'd expect.

    With named parameters, it can be easy to misspell keys in your method
    definition. For example:

      my $win = [
        $args->{year},
        $args->{race},
        exists($args->{jockee}) ? $args->{jockey} : (),
      ];

    Note "jockee"! This can lead to hard-to-find bugs. There's a
    "compile_named_oo" function which may help and can lead to cleaner code.

      package Horse {
        use Types::Common::Numeric qw( PositiveInt );
        use Types::Common::String qw( NonEmptyStr );
        use Type::Params qw( compile_named_oo );
        ...;

        sub add_win {
          state $check = compile_named_oo(
            year    => PositiveInt,
            race    => NonEmptyStr,
            jockey  => NonEmptyStr, { optional => 1 },
          );

          my $self = shift;
          my $args = $check->(@_);
          my $win = [
            $args->year,
            $args->race,
            $args->has_jockey ? $args->jockey : (),
          ];
          push @{ $self->wins }, $win;
          return $self;
        }
      }

    Now $args is a blessed object that you can call methods on. There is of
    course a performance penalty for this, but it's surprisingly small.

    For more information on Type::Params, and third-party alternatives, see
    Type::Tiny::Manual::Params.

NEXT STEPS
    Congratulations! I know this was probably a lot to take in, but you've
    covered all of the essentials.

    You can now set type constraints and coercions for attributes and method
    parameters in Moo! You are familiar with a lot of the most important and
    useful type constraints and understand parameterization and how it can
    be used to build more specific type constraints.

    (And I'll let you in on a secret. Using Type::Tiny with Moose or Mouse
    instead of Moo is exactly the same. You can just replace "use Moo" with
    "use Moose" in any of these examples and they should work fine!)

    Here's your next step:

    *   Type::Tiny::Manual::UsingWithMoo2

        Advanced use of Type::Tiny with Moo, including unions and
        intersections, "stringifies_to", "numifies_to",
        "with_attribute_values", and "where".

NOTES
    On very old versions of Moo "coerce => 1" is not supported. Instead you
    will need to provide a coderef or object overloading "&{}" to coerce.
    Type::Tiny can provide you with an overloaded object.

      package Horse {
        use Moo;
        use Types::Standard qw( Str Num ArrayRef Bool );
        use namespace::autoclean;

        ...;  # name, gender, age, children, wins
        has is_alive => (
          is      => 'rw',
          isa     => Bool,
          coerce  => Bool->coercion,  # overloaded object
        );
      }

    If you have a very old version of Moo, please upgrade to at least Moo
    1.006000 which was the version that added support for "coerce => 1".

AUTHOR
    Toby Inkster <tobyink AT cpan.org>.

COPYRIGHT AND LICENCE
    This software is copyright (c) 2013-2014, 2017-2021 by Toby Inkster.

    This is free software; you can redistribute it and/or modify it under
    the same terms as the Perl 5 programming language system itself.

DISCLAIMER OF WARRANTIES
    THIS PACKAGE IS PROVIDED "AS IS" AND WITHOUT ANY EXPRESS OR IMPLIED
    WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
    MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE.