WASH—Web Authoring System in Haskell User Manual
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1. Element An element node represents an XML element it corresponds to the text between a start tag and its matching end tag Each element node has attached to it a number of attribute nodes and an ordered list of child nodes Child nodes may be of type Text Element or Comment Attribute An attribute node represents an XML attribute occurrence Text A text node represents a piece of text without markup Comment A comment node represents a comment in XHTML which is not displayed Many XML standards e g XPath deal with documents in terms of sequences of document nodes and this notion also provides the right approach for WASH 2 2 3 Document templates The above Hello World program is extremly unflexible because the entire text is hard coded into the program Fortunately it is easy to abstract over parts of the document s text by using a Haskell function to construct the document and a text escape lt gt in the constructed page The text escape may contain an arbitrary Haskell expression of type String mypage ttl lt html gt lt head gt lt title gt lt tt1 gt lt title gt lt head gt lt body gt lt hi gt lt tt1 gt lt h1 gt lt body gt lt html gt mainCGI ask mypage Hello World To add further contents to such a page requires abstraction over sequences of document nodes because such a sequence is expected in the body of the docu ment after the header hi The preprocessor su2. lt table gt lt gt collectOne F3 emailH firstH lastH let email unEmailAddress value emailH firstname unNonEmpty value firstH lastname unNonEmpty value lastH in 12 2 4 Menu style interaction Many programs are structured around menus The interaction with such a program comprises navigation through nested menus selection of a task in the menu and then returning to the menu again A long interaction sequence with this kind of program may lead to an inefficiency in the current implementation of WASH The implementation stores an interaction log of all input values and the results of all I O operations in an invisible field in the generated Web page If this log grows too big the response time may suffer considerably To avoid this inefficency WASH provides two operators once Read a Show a gt CGI a gt CGI a forever CGI O gt CGI O The operator once summarizes a sequence of CGI actions into a single one which only contributes a single value to the log A typical use is the following userInteraction user do once oneAction user userInteraction user where the function oneAction presents the menu and then performs the selected choice The end of processing the selected choice is indicated in the program by return Executing this action returns the through once Example ex3 2 a hs contains a small but full example The above pattern occurs so frequently that it has been a
3. multiply F2 factor rept let n value factor r value rept in Often a button just serves as a continuation button so that its callback does not require a handle argument Such a button is created with submitO CGI gt HTMLField x y 2 3 3 Unvalidated input fields The types of all constructors of input fields follow the same pattern type HTMLField x y inputhandle WithHTML x CGI gt WithHTML y CGI inputhandle That is they take a sequence of attribute nodes as their argument and return a singleton document action for the input element The latter action yields the input handle of type inputhandle The main difference between the fields is the type of the inputhandle textInputField HTMLField x y InputField String INVALID The value returned by the field s handle is a String makeTextarea String gt HTMLField x y InputField String INVALID This function creates a multi line textual input box with a preset string specified with the function s argument Its handle returns a value of type String checkboxInputField HIMLField x y InputField Bool INVALID A checkbox generates an input field of type Bool indicating whether the box was checked makeButton HTMLField x y InputField Bool INVALID Similar to the above but using a button element instead of input In conse quence the argument is not restricted to attributes but nested XHTML elements may be used to specify the surface of the
4. WASH Web Authoring System in Haskell User Manual Peter Thiemann August 18 2004 Abstract This user manual fulfills two purposes First it contains a gentle in troduction to the concepts of WASH by guiding through the construction of an example application After studying this introduction the reader should be ready to understand the example applications shipped with the system and to get further ideas from studying the automatically generated reference manual Second it gives some hints how to tackle frequently occurring tasks Most of them are also covered by one or more example applications 1 Overview Currently WASH consists of five parts four of which are visible e WASHCGI a package that contains the functionality to develop CGI based interactive Web applications It requires the packages WASHHTML and Utility e WASHHTML an API for constructing XHTML output It requires the package Utility e WASHMail an API for composing and decomposing multi part emails and for sending mails e Wash2hs a preprocessor that provides a JSP style surface syntax for WASHHTML The preprocessor should be considered a front end for XHTML generation with WASHHTML The surface syntax that it provides simplifies the latter task con siderably It is particularly helpful when designing forms with more than a few input fields In addition using the preprocessor is not a yes or no question Modules using the WASH surface synta
5. lt div gt showResult iName let name value iName in standardQuery Hello lt gt Hello lt Z name gt how are you lt gt In the example textInputField empty creates a standard textual input field In place of empty attributes for the input element may be supplied The no tation iName lt in the code escape binds the handle to the input field to the variable iName The function submit creates a submission button on the web page Its first parameter iName is the handle processed by this button The second parameter showResult is the callback function that is applied to the handle when the button is clicked The third parameter empty supplies further attributes to the submission button none in this case The callback function applies the function value to the handle to extract its string value into the variable name Then it constructs and displays an answer page using the input value in a string escape Due to typing restrictions it is not possible to extract the value from a handle before it is passed to a callback function Finally to pass more than one handle to a callback requires the use of special tuple constructors F2 F3 F4 and so on for two three four etc handles For example submit F2 factor rept multiply fieldVALUE START applies the callback multiply to two handles factor and rept The same pattern must be used in the definition of the callback to take the handles apart again
6. some IO action returning String standardQuery Haskell Library Modules do sg lt selectionGroup let makeRow row selectionDisplay sg at row dispRow dispRow button texts tr do td button sequence Prelude map td texts p table mapM makeRow 0 as_rows at 1 submit sg page2 empty page2 sg let row unAR value sg in standardQuery Selected row do text You selected text show row The function dispRow takes the button and a list of texts and builds a row of an XHTML table from it 15 3 3 Non XHTML output The combinator tell CGIOutput a gt a gt CGI delivers any output that may sensibly be returned by a Web application The most interesting of these are XHTML elements strings and FileReferences Further output options are Location and Status responses as defined in the CGI standard The reference manual contains a full listing 3 4 Defining new types for validated input fields A type suitable for entering through inputField must be a member of the type classes Reason and Read The class Reason has a single member function class Reason a where reason a gt String This member function should return a description of the input syntax for the type Its value is used to generate the error message and the explanatory text for an input field of this type To add new instances to Reason requires to import module Fields The class Read is predefined in Haskell To create an instance
7. 8 encoding of the document itself The presence of an input field for file uploading in the form changes the encoding type to multipart form data 3 7 More than one form on a page This is not necessary with WASH Grouping your input fields by passing them in groups to the submit functions is sufficient 3 8 Encryption of the interaction state Each XHTML page delivered by WASH contains a log for reconstructing the current interaction state In standard operation this log is only superficially encoded using the Base64 encoding If it is expected that the log contains sensitive information then further encoding is advisable WASH can be switched to encode logs by providing a key file at tmp KEYFILE This location is configurable in module CGIConfig This file should be a large file with random content For encryption WASH generates a random offset into the key file and adds the log bytewise to the contents of the keyfile and to the accumulated sum of the log so far see functions encrypt1 and decrypt1 in module RawCGIInternal for details The security of this mechanism relies entirely on the key file If it is too small its contents is not really random or the same file is used for too long then it is easy to break the encryption 2The latter behavior is not fixed in any standard known to the author 17 4 Installing and Trouble Shooting 4 1 Compiling a WASH program This section assumes the following setup e All fil
8. button fileInputField HTMLField x y InputField FileReference INVALID The returned handle contains a value of type FileReference Such a value indicates a file that has been uploaded and it contains its temporary location and some information about it e g its mediatype There are no guarantees about the lifetime of this temporary file so an application is advised to move the file to a safe location as soon as possible The attentive reader may be wondering why the type of each input field contains INVALID The type INVALID indicates that the handle is not yet filled with data Hence the function value which requires a VALID in place of the INVALID in the type of the handle is not applicable to a freshly created handle Only the submit function converts an INVALID handle into a VALID one This setup ensures that handles are not read out before they are filled and it also provides a convenient place where additional checks on the input may be performed 2 3 4 Wrapped input fields For a few types of input fields WASH provides a more high level way of specifying them than XHTML These are radio buttons and selection boxes Radio buttons In XHTML radio buttons are implicitly grouped through sharing a common name Since WASH does not require naming of input fields such grouping is not possible Hence WASH has an abstract notion of a radioGroup an invisible widget which is created first and to which all but tons of the group are at
9. its body document Usually no further configuration of the form is required standardQuery uses the function makeForm to construct the form It be haves like any other element constructor It should not be used in the body of standardQuery 2 3 2 Input philosophy Each input element is created by a dedicated function that creates a singleton sequence with the desired element and returns a handle for the input field The handle makes the usual naming of each input element with a string unnec essary Instead the handle is a value bound to a program variable and there are functions to access its value in the program Submission buttons are special input elements that cause the values of all input fields to be gathered and sent to the object and method specified in the form element WASH attaches a callback and some input handles to each submission button Pressing the button invokes the callback on the specified input handles In addition it is possible to attach a validation to each input element This validation is performed whenever the callback is invoked but only for its input handles The system automatically rejects invalid inputs and redisplays the previous page with the erronous input indicated and explained Here is a simple example mainCGI standardQuery Your name please lt div gt lt p gt Enter your name lt iName lt textInputField empty gt lt p gt lt p gt lt submit iName showResult empty gt lt p gt
10. of Read a requires the definition of a function type ReadS a String gt a String readsPrec Int gt ReadS a ReadS a is the type of a parser that converts strings into values of type a Such parsers may be created with functions from the module Utility SimpleParser which provides the usual monadic interface for parsing Parsers may also be created by hand as the following example excerpted from the library s source demonstrates Non empty strings newtype NonEmpty NonEmpty unNonEmpty String instance Read NonEmpty where readsPrec i 0 readsPrec i str NonEmpty str instance Reason NonEmpty where reason _ non empty string 3 5 Character set encoding Early versions of WASH have ignored this surprisingly tricky issue The current version assumes that the I O primitives use values of type Char between 0 and 16 255 to read and write bytes It generates output in UTF 8 format whenever that is covered by an applicable standard XHTML Email and it expects the form submissions from the browsers also in UTF 8 encoding This choice causes older browsers to choke and particularly email using the UTF 8 encoding causes problems with newer email clients 3 6 Form attributes The standard setting of the form attributes is as follows e enctype application x www form urlencoded e method post e action full url of the script itself e accept charset UTF 8 implicit through the UTF
11. attribute nodes from the bracketed list of name value pairs For example the value of twoAttrs in twoAttrs lt bgcolor red onload init gt is a sequence of two attribute nodes Also lt gt provides a further way of constructing the empty sequence A code escape in child position attaches attributes to an element For ex ample in lt body gt lt twoAttrs gt lt body gt the two attributes are attached to the body element A code escape with attribute nodes may also be used in attribute position lt body lt twoAttrs gt gt lt body gt In addition only the attribute value may be replaced by a code escape as in bgcolor red onload init b lt body bgcolor lt bgcolor gt onload lt onload gt gt lt body gt In this case the type of the code escape must be String 2 2 7 The API for constructing document nodes We have seen XML notation for creating singleton sequences of element nodes attribute nodes and implicitly text nodes All such nodes can be created by accessing the underlying API directly This notation may be mixed arbitrarily with using the XML notation Element nodes for the tag name may be created with the function name WithHTML x cgi a gt WithHTML x cgi a The argument of the tag function is the sequence of child nodes for the element and the result is a singleton sequence with element name A function name is predefined for every element name ava
12. bstracted into the function forever userInteraction user forever oneAction user Its implementation is even more efficient than the recursive definition of userInteraction 3 Miscellaneous This section collects various comments and properties of WASH that do not fite lsewhere 3 1 Accessing I O Since the main thread of execution in a WASH program takes place in a CGI action the standard Haskell I O operations are not directly applicable They need to be wrapped into the operator io io Read a Show a gt 10 a gt CGI a The typing of I O actions suggests that they be used before using ask standardQuery or similar operators 13 The use of I O actions with bulky output is possible but strongly discour aged If at all possible multiple I O actions should be gathered to a single action and the value returned by the action should be as small as possible Some times a value can be made smaller by incorporating part of the computation into the I O action For example a naive implementation of password verification is pwfile lt io readFile passwords let logged_in checkpasspw pwfile login passwd where checkpasspw returns True or False A much more efficient implemen tation is logged_in lt io do pwfile lt readFile passwords return checkpasspw pwfile login passwd 3 2 Bulky I O Sometimes it cannot be avoided to have an I O action return a large amount of data For example the resu
13. current implementation cannot deal efficiently with bulky output It works but things become slow 14 We will discuss the group of operations for selecting single rows the opera tions for multiple rows work analogously As with radio buttons which are used in the implementation of selection groups we first need to create a selection group using selectionGroup WithHTML y CGI SelectionGroup AR INVALID Later on the function selectionButton SelectionGroup AR INVALID gt AT gt Int gt HTMLField x y takes a selection group an abstract table and a valid row number in the table and places a radio button on the page Clicking the button sets the value of the selection group to the contents of the row associated with the button The function selectionDisplay SelectionGroup AR INVALID gt AT gt Int gt WithHTML x CGI gt WithHTML x CGI gt WithHTML x CGI a gt WithHTML x CGI a combines the creation of the button with the extraction of the text nodes from the abstract table The first three parameters determine the selection group the abstract table and the row number The last parameter is a function that takes first the button and then the list of text nodes corresponding to the contents of the row The job of this function is to combine these to an XHTML fragment for displaying the row s contents and the button Here is an example for using an abstract table pagel do at lt table_io
14. es are in one directory e The main program resides in file Main hs e Each subsidiary modules reside in file modname hs Place the following Makefile in the directory HC ghc HCFLAGS package WASHCGI HMAKE HC HCFLAGS make RM rm f HSFILES wildcard hs Main HSFILES HMAKE clean RM Main o hi h hs HMAKE o hs HC HCFLAGS c lt o hi o Type gnumake to compile the program Main It requires 1 the preprocessor wash2hs installed somewhere in your search path 2 the Haskell compiler ghc installed somewhere in your path The name of the executable program is Main To use it as a CGI program just copy it into a CGI enabled directory in the document tree of your web server No further configuration is required but certain directories need to be readable and writable by the effective user running the CGI programs see Sec 4 2 To have more than one main program in one directory each program needs to have its own main module Each main module resides in a file by itself with an arbitrary file name and extension hs for example MyMod hs The makefile above is equipped to compile such a program by entering 18 gnumake MyMod This creates an executable named MyMod which may then be passed to the Web server in the usual way 4 2 Files directories and permissions WASHCGI uses the following directories which can be modified in module CGIConfi
15. g Wrong permissions for these directories is a frequent source of problems tmp Images r w tmp Frames r w tmp Persistent2 r w tmp REGISTRY r w tmp KEYFILE r if CGIGraphics is used temporary files if frames are used experimental temporary files if module Persistent2 is used PERSISTENT FILES always temporary files if log encryption is used provided by user If running under a real user e g when started from the command line then the user s home directory is prefixed to the path name The following programs are assumed usr X11R6 bin ppmtogif x usr X11R6 bin pbmtext x usr X11R6 bin pnmcrop x usr X11R6 bin pbmtopgm x usr X11R6 bin pgmtoppm x bin cat x usr sbin sendmail x 19 if CGIGraphics used if CGIGraphics used if CGIGraphics used if CGIGraphics used if CGIGraphics used for displaying files and FileReferences if WashMail used for sending mail
16. ilable in XHTML For example the explicit Hello World program might look like this mainCGI ask html do head title text Hello World body hi text Hello World Attribute nodes may also be created with the function attr String gt String gt WithHTML x cgi The first argument is the attribute name and the second is its value There are no restrictions on the value The function takes care of all necessary escaping Text and comment nodes may be created with the functions text String gt WithHTML x cgi comment String gt WithHTML x cgi In both cases the string argument is the text as it should appear on the screen Both function automatically escape special characters like lt gt and amp to avoid unwanted markup in the generated document The comment function may insert extra characters to avoid the premature end of the comment 2 3 Forms and input fields The first step in making an application interactive is defining input facilities They are provided via the various input fields defined in XHTML All input fields in an XHTML document must appear inside of a form element Attributes of the latter specify an object an URL and a method GET or POST that should be invoked with the parameters entered in the input fields contained in the form 2 3 1 Creating a form The function standardQuery already wraps a form with a suitable specification of an object and a method around
17. ipulating document actions Often it is necessary to build documents from document fragments While code escapes provide the means of parameterizing one fragment by another sometimes documents need to be glued together for example to construct lists in the output Since a document action may be regarded as a sequence of document nodes WASHHTML provides the operations for creating an empty sequence or a one element sequence a singleton and for concatenating sequences There is inten tionally no way of examining document sequences once they are constructed They can only be converted to the textual representation of a sequence of XHTML elements The empty sequence may be created in two ways Either with lt gt lt gt or with empty Any use of an XHTML element creates a singleton sequence For example lt hi gt Hello World lt h1 gt creates a sequence with exactly one h1 node Two or more document sequences may concatenated using the do notation provided by Haskell do seqi seq2 do seqi seq2 seqn or seqn In the second variant the layout is essential the first character of seq1 seq2 seqn must be aligned to the same starting column Alternatively the binary operators and gt gt may be used to concatenate sequences 2 2 6 Attributes Attributes nodes can be created inline using their standard XHTML syntax Additionally the syntax lt gt creates a sequence of
18. lts of a database query may be expressed with an I O action that returns a list of lists of strings where the nested list comprises the tuples selected by the query The problem with an I O action is that the rest of the interaction may depend on the result of the action This potential dependency requires the current implementation of WASH to store results of I O actions In many cases however this assumption is overly pessimistic Often data is only displayed and the rest of the interaction depends on a single tuple at most Such cases are served best with an abstract table An abstract table is created by a variant of the io operator table_io I0 String gt CGI AT It converts an I O action that returns a list of database tuples encoded as above into a CGI action that returns an abstract table of type AT The value stored in an abstract table cannot be read out directly However each component in a abstract table can be turned into a text node and thus incorporated into a generated XHTML page This is accomplished with the functions as_rows AT gt Int as_cols AT gt Int getText Monad m gt AT gt Int gt Int gt WithHTML x m for determining the number of rows and columns and for obtaining a text node that corresponds to an entry in a specific row and column of the table Furthermore there are two kinds of selection facilities that may be used to extract entire rows tuples from the table lThe
19. on echoing variant Apart from their typing properties they are used in exactly the same way as their unvalidated 11 variants In addition instead of yielding a string value those handles yield a value of type a It is important that the callback function that consumes such a typed in put handle accesses its value in a way that makes its type obvious This may require a type specification in the callback function as the following example demonstrates mainCGI standardQuery Multiplication Drill table do factor lt question Choose a factor 2 rept lt question Number of exercises 20 tr td submit F2 factor rept multiply fieldVALUE START multiply F2 factor rept let n r Int n value factor r value rept in multiplyBy n r 1 O Often the accessor functions for the value fixes the type sufficiently This is the case in the example below for the predefined Email and NonEmpty field types lt gt lt table gt lt tr gt lt td gt Email Adresse lt td gt lt td gt lt emailH lt inputField fieldSIZE 40 gt lt td gt lt tr gt lt tr gt lt td gt Vorname lt td gt lt td gt lt firstH lt inputField fieldSIZE 40 gt lt td gt lt tr gt lt tr gt lt td gt Nachname lt td gt lt td gt lt lastH lt inputField fieldSIZE 40 gt lt td gt lt tr gt lt tr gt lt td gt lt submit F3 emailH firstH lastH collectOne empty gt lt td gt lt tr gt
20. ond argument specifies an optional preselection Nothing means no item is preselected Just x means that value x is preselected The third argument specifies the list of all choices The result is an input field as before Here is an example use displayCard 0 MasterCard displayCard 1 Visa displayCard 2 American Express lt tr gt lt td gt My Credit Card is a lt ccF lt selectSingle displayCard Nothing 0 1 2 empty gt lt td gt lt tr gt There is a shortcut selectBounded for simple selection boxes when the choice type is a finite enumerated type Multiple selection boxes are accessible through selectMultiple Their in terface is slightly more complicated but the essence is as with selectSingle 2 3 5 Validated input fields The textual input fields come in a variant that validates all input and converts it to a specific type That is a field can be restricted to only accept numbers booleans dates credit card numbers or email addresses Also simple restric tions like non emptiness of a field can be enforced The validation mechanism is extensible in the sense that new fields with customized restrictions and return types may be specified in the program Validated and typed input fields are created by inputField Reason a Read a gt HIMLField x y InputField a INVALID passwordInputField Reason a Read a gt HTMLField x y InputField a INVALID with passwordInputField supplying the n
21. pports it through a code escape lt h gt The code escape may contain arbitrary Haskell code for document ac tions using Haskell syntax or XHTML notation Of course such nested XHTML fragments may contain arbitrarily nested escapes themselves yourpage ttl contents lt html gt lt head gt lt title gt lt tt1 gt lt title gt lt head gt lt body gt lt hi gt lt tt1 gt lt h1 gt lt Z contents gt lt body gt lt html gt mainCGI ask yourpage Hello World lt gt lt gt Since we don t have a useful content for the body of the template at this point we pass an empty sequence as parameter to yourpage In general the markers lt gt and lt gt delimit a nameless container for a sequence of document nodes 2 2 4 A standard query operation The combination of yourpage and ask is so universally useful that WASH al ready provides it as a function standardQuery String gt WithHTML x CGI a gt CGI Its first argument is a title string which is put in the title line of the browser window and which makes up the header line of the page The second argument is a document action In addition standardQuery wraps the body of the document into a form tag so that input elements may be used freely within the page Using standardQuery the Hello World for WASH completes the template above by one line mainCGI standardQuery Hello World lt gt lt gt 2 2 5 Man
22. program First and foremost the WASH main program is a Haskell main program That is there must be a module named Main residing in file Main hs and it must define a value main IO For a WASH program the following template is used to define main module Main where import CGI hiding div head map span main run mainCGI mainCGI e Every module using WASH needs the line import CGI e The hiding clause in the import avoids name clashes between names de fined in the CGI module and the standard prelude div is integer division head map and span are frequently used functions on lists e The value mainCGI is a CGI action of type CGI CGI actions can be constructed with the functions described below and composed with the standard monad operations expressing sequencing of actions 2 2 Generating XHTML output The standard way of creating an XHTML output page is using the function ask WithHTML x CGI a gt CGI Its argument is a document action Its type looks rather complicated but for the moment it is sufficient to note that this is exactly the type of value created by the WASHHTML API and also by the element notation provided through the preprocessor The latter is easier on persons familiar with XHTML and is applicable in most occasions Hence we defer the discussion of the API because its direct use is only required in exceptional cases 2 2 1 A Hello World page The simplest Web script jus
23. t produces a static output page whenever it is run Thanks to the preprocessor it suffices to apply the ask combinator to an XHTML document mainCGI ask lt html gt lt head gt lt title gt Hello World lt title gt lt head gt lt body gt lt hi gt Hello World lt h1 gt lt body gt lt html gt To run this program the definition of mainCGI should be entered in the template for the Main module and placed in a file named Main hs The transformation to Haskell proper is performed by the compiler with the command ghc package WASH make Main The resulting binary program file is called Main It may be run from the com mand line to look at the HTML output or the binary may be copied into a CGlI enabled directory in the directory tree of a Web server often cgibin Start it from your favorite Web browser by entering its url http myserver cgibin Main The name myserver the path cgibin and the name in particular the suffix you have to use for the program itself depends on the configuration of the Web server you are using Contact the server s administrator if in doubt 2 2 2 Digression the XML document model Proficient use of WASH requires a little knowledge of the XML document model An XML document is represented by a tree where the nodes of the tree contain different kinds of information The DOM standard distinguishes twelve different types but for purposes of XHTML generation four types are sufficient
24. tached later on radioGroup Read a gt WithHTML x CGI RadioGroup a INVALID radioButton Show a gt RadioGroup a INVALID gt a gt HTMLField x y A handle is only created for the radio group Each individual button only specifies a value of a suitable type a This type a may be anything that is Readable and Showable for exampe numbers strings etc However all buttons of a radio group must yield values of the same type Here is an example using radio buttons where PayCredit and PayTransfer are two values of an enumerated type lt table gt lt rg lt radioGroup gt lt tr gt lt td gt lt radioButton rg PayCredit empty gt lt td gt lt td gt Pay by Credit Card lt td gt lt tr gt lt tr gt lt td gt lt radioButton rg PayTransfer empty gt lt td gt lt td gt Pay by Bank Transfer lt td gt lt tr gt lt tr gt lt td gt lt td gt lt td gt lt submit rg paymentOption empty gt lt td gt lt tr gt lt table gt 10 Selection boxes A simple selection box supplies a choice of one from many items It is implemented by the function selectSingle selectSingle Eqa gt a gt String gt Maybe a gt a gt HTMLField x y InputField a INVALID The type a is the type of values returned from the selection box It only needs to provide an equality Eq a The first argument is a function that turns a selection value into a string for displaying the choices on the screen The sec
25. x can be freely interchanged with ordi nary modules On the down side errors get a bit harder to localize with the preprocessor but this is far outweighed by the simplified interface WASH is built on top of the purely functional programming language Haskell The embedding is fairly unobtrusive and the presence of the preprocessor allows the construction of simple applications without knowing much about Haskell Thus simple task are simple and complicated task require some knowledge of the language WASH relies heavily on monads A monad is an abstraction that allows side effects to be introduced in Haskell programs For understanding WASH the simple intuition of a monad as packaging a sequence of actions is sufficient However it is recommended to acquire some background knowledge on monads to get the full potential of the APIs and to progress beyond casual use An essential prerequisite to using WASH is knowledge of XHTML An ap propriate characterization of WASH is programming the Web like writing an interactive application with an XHTML GUI While WASH takes away many tedious tasks like administering names of input elements and managing forms there is still the work of creating the actual layout 2 Getting started This section covers the essential steps to create a simple interactive application The first step comprises the magic words for creating a WASH program The next step is the generation of XHTML output 2 1 The main
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