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That said, most Clojure programs begin life as text files, and it is the task of the reader to parse the text and produce the data structure the compiler will see. This is not merely a phase of the compiler. The reader, and the Clojure data representations, have utility on their own in many of the same contexts one might use XML or JSON etc.
One might say the reader has syntax defined in terms of characters, and the Clojure language has syntax defined in terms of symbols, lists, vectors, maps etc. The reader is represented by the function read, which reads the next form (not character) from a stream, and returns the object represented by that form.
Since we have to start somewhere, this reference starts where evaluation starts, with the reader forms. This will inevitably entail talking about data structures whose descriptive details, and interpretation by the compiler, will follow.
The behavior of the reader is driven by a combination of built-in constructs and an extension system called the read table. Entries in the read table provide mappings from certain characters, called macro characters, to specific reading behavior, called reader macros. Unless indicated otherwise, macro characters cannot be used in user symbols.
Metadata is a map associated with some kinds of objects: Symbols, Lists, Vector, Sets, Maps, tagged literals returning an IMeta, and record, type, and constructor calls. The metadata reader macro first reads the metadata and attaches it to the next form read (see with-meta to attach meta to an object):
^:a 1 :b 2 [1 2 3] yields the vector [1 2 3] with a metadata map of :a 1 :b 2.
The key in each pair is a tag that will be recognized by the Clojure reader. The value in the pair is the fully-qualified name of a Var which will be invoked by the reader to parse the form following the tag. For example, given the data_readers.clj file above, the Clojure reader would parse this form:
by invoking the Var #'my.project.foo/bar on the vector [1 2 3]. The data reader function is invoked on the form AFTER it has been read as a normal Clojure data structure by the reader. For your own data reader functions, you should report errors by throwing instances of RuntimeException with messages providing error information.
Reader tags without namespace qualifiers are reserved for Clojure. Default reader tags are defined in default-data-readers but may be overridden in data_readers.clj / data_readers.cljc or by rebinding *data-readers*. If no data reader is found for a tag, the function bound in *default-data-reader-fn* will be invoked with the tag and value to produce a value. If *default-data-reader-fn* is nil (the default), a RuntimeException will be thrown.
Clojure 1.4 introduced the instant and UUID tagged literals. Instants have the format #inst "yyyy-mm-ddThh:mm:ss.fff+hh:mm".NOTE: Some of the elements of this format are optional. See the code for details.The default reader will parse the supplied string into a java.util.Date by default. For example:
Since *data-readers* is a dynamic var that can be bound, you can replace the default reader with a different one. For example, clojure.instant/read-instant-calendar will parse the literal into a java.util.Calendar, while clojure.instant/read-instant-timestamp will parse it into a java.util.Timestamp:
If no data reader is found when reading a tagged literal, the *default-data-reader-fn* is invoked. You can set your own default data reader function and the provided tagged-literal function can be used to build an object that can store an unhandled literal. The object returned by tagged-literal supports keyword lookup of the :tag and :form:
In cases where is not feasible to isolate the varying parts of the code, or where the code is mostly portable with only small platform-specific parts, 1.7 also introduced reader conditionals, which are supported only in cljc files and at the default REPL. Reader conditionals should be used sparingly and only when necessary.
The syntax for #?@ is exactly the same but the expression is expected to return a collection that can be spliced into the surrounding context, similar to unquote-splicing in syntax quote. Use of reader conditional splicing at the top level is not supported and will throw an exception. An example:
The read and read-string functions optionally take a map of options as a first argument. The current feature set and reader conditional behavior can be set in the options map with these keys and values:
If the reader is invoked with :read-cond :preserve, the reader conditional and non-executed branches will be preserved, as data, in the returned form. The reader-conditional will be returned as a type that supports keyword retrieval for keys with :form and a :splicing? flag. Read but skipped tagged literals will be returned as a type that supports keyword retrieval for keys with :form and :tag keys.
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Note: Apps that let people access digital content such as music or video, but not as the primary functionality, are not considered reader apps and are not eligible for the External Link Account Entitlement. For example, a social networking app that lets people stream audiovisual content is not eligible.
In addition to enabling an entitlement for your app and implementing the required External Link Account APIs, you must follow usage requirements designed to help protect privacy and security, prevent scams and fraudulent activity, and maintain the overall quality of the experience. Any link provided in a reader app must:
The External Link Account Entitlement can be used by approved reader apps distributed in any country or region where the App Store is available, and is compatible with devices running iOS and/or iPadOS, or tvOS 16.4 or later.
No. The External Link Account Entitlement is optional and only required for reader apps that wish to link out to a website for account creation and management. Reader apps can continue to offer account creation for free tiers, and account management functionality for existing customers within the app, per guideline 3.1.3(a).
A reader account enables data consumers to access and query data shared by the provider of the account, with no setup or usage costs forthe consumer, and no requirements for the consumer to sign a licensing agreement with Snowflake.
The reader account is created, owned, and managed by the provider account, which assumes all responsibility for credit charges incurred byusers in the reader account. Similar to standard consumer accounts, the provider account uses shares to share databases with readeraccounts; however, a reader account can only consume data from the provider account that created it.
Because a reader account does not have a licensing agreement with Snowflake, support services are not available to the general users in the account. Instead, as the provider of the account, youfield questions and requests from users in the account and respond as appropriate.
If you are unable to directly answer their questions or resolve their requests/issues, you can open a Snowflake Support ticket through the normal channels (as outlined in your support agreement).Once a response has been provided by Snowflake Support, you then communicate the information back to the appropriate users in the reader account.
If you have the ACCOUNTADMIN role (or have a role that has been granted the CREATE ACCOUNT privilege), you can use Snowsight orClassic Console to perform most tasks related to creating and managing reader accounts.
By default, only users with the ACCOUNTADMIN role can create reader accounts and therefore, as the owner of the account, manage the accounts. To support delegating these tasks to other users, theCREATE ACCOUNT global privilege can be granted to other roles (system-defined or custom). Then, users with the role can create reader accounts and perform all tasks associated with managing theaccounts created using the role.
This command can be used to monitor the total number of reader accounts for your account. If the total number reaches the limit (20), you may need to drop some accounts or contactSnowflake Support to request the limit be increased.
In addition, you can use the views in the READER_ACCOUNT_USAGE schema (in the SNOWFLAKE shared database) to query information about the reader accounts created for your account. For more details, seeAccount Usage.
The Dialect class is a container class whose attributes containinformation for how to handle doublequotes, whitespace, delimiters, etc.Due to the lack of a strict CSV specification, different applicationsproduce subtly different CSV data. Dialect instances define howreader and writer instances behave.
To make it easier to specify the format of input and output records, specificformatting parameters are grouped together into dialects. A dialect is asubclass of the Dialect class having a set of specific methods and asingle validate() method. When creating reader orwriter objects, the programmer can specify a string or a subclass ofthe Dialect class as the dialect parameter. In addition to, or insteadof, the dialect parameter, the programmer can also specify individualformatting parameters, which have the same names as the attributes defined belowfor the Dialect class.
There are a lot more key commands available for each screen reader. The commands in this post are not intended to be a comprehensive tutorial, but they will help you navigate a website to get a sense of how things work. If possible organise some usability testing with screen reader users (and people with other types of disability) to get a real understanding of how people experience your website.
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