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   1                                MHL Design
   2
   3 Mhl is a formatted  message  listing  program.  It  can  be  used  as  a
   4 replacement  for  "c"  (the default message lister).  As with c, each of
   5 the messages specified as arguments (or stdin) will be output.  If  more
   6 than  one  message file is specified, the user will be prompted prior to
   7 each one, and a <return> or <eot> will begin the output,  with  <return>
   8 clearing  the  screen (if appropriate), and <eot> suppressing the screen
   9 clear.  An <interrupt> will abort the current message  ouput,  prompting
  10 for  the next message (if there is one), and a <quit> will terminate the
  11 program (without core dump).
  12
  13 NOTE: This is a preliminary document and the design is VERY young.
  14
  15 Syntax: mhl [switches] [files]
  16
  17 As in all MH programs, mhl looks for a line "mhl: args"  in  the  user's
  18 profile, and thus allows tailored defaults.  The switches are:
  19
  20         -clear          /* Clear screen each page       */
  21         -noclear        /* Don't clear screen each page */
  22         -folder folder  /* Use this "folder" name       */
  23         -form formfile  /* Name of the format file      */
  24         -length         /* Screen length                */
  25         -width          /* Screen width                 */
  26         -help           /* Standard help message        */
  27
  28 All of the functions these switches perform are affected  or  controlled
  29 by information elsewhere.  That is, the format file can specify "clear",
  30 in which case, the  command  line  switches  will  override.  Also,  the
  31 length  and  width  can  be  specified in the format file, or default to
  32 40x80.  The folder is used in constructing a message name  (see  special
  33 component  "MessageName" below).  If it is not specified in a switch, it
  34 is taken from the environment variable  "mhfolder",  which  show,  next,
  35 prev, and pick initialize appropriately.
  36
  37 If  the  form  file  is  not  specified,  it  is  taken  from  the  file
  38 "mhl.format" in the user's MH directory, and if that file doesn't exist,
  39 it is taken from "/etc/mh/mhl.format". (Same evaluation hierarchy as the
  40 compose form file.)
  41
  42 Mhl operates in two phases: 1) read and parse the format  file,  and  2)
  43 process each message (file).  During phase 1, an internal description of
  44 the format is produced as a structured list.  In phase 2, this  list  is
  45 walked  for each message, outputing message information under the format
  46 constraints from the format file.
  47
  48 The "mhl.format" file contains information controlling screen  clearing,
  49 screen  size,  wraparound control, transparent text, component ordering,
  50 and component formatting.  Also, a list of components to ignore  may  be
  51 specified, and a couple of "special" components are defined  to  provide
  52 added  functionality.  Message  output will be in the order specified by
  53 the order in the format file.
  54
  55
  56 Each line of mhl.format has one of the formats:
  57
  58         ;comment
  59         :cleartext
  60         variable[,variable...]
  61         component:[variable,...]
  62
  63 A line beginning with a ";"  is  a  comment,  and  is  ignored.  A  line
  64 beginning with a ":" is clear text, and is output exactly as is. (A line
  65 containing only a ":" produces a blank  line  in  the  output.)  A  line
  66 beginning  with  "component:"  defines  the  format  for  the  specified
  67 component, and finally, remaining lines define the global environment.
  68
  69 For example, the line:
  70
  71 width=80,length=40,clearscreen,overflowtext=***,overflowoffset=5
  72
  73 defines the screen size to be 80 columns by 40 rows, specifies that  the
  74 screen  should  be  cleared  prior  to  each  page,  that  the  overflow
  75 indentation is 5, and that overflow text should be flagged with "***".
  76
  77 Following are all of the current  variables  and  their  argmuments.  If
  78 they  follow  a component, they apply only to that component, otherwise,
  79 their affect is global.  Since the whole format  is  parsed  before  any
  80 output processing, the last global switch setting for a variable applies
  81 to the whole message.
  82
  83 width=#         Screen width, component width
  84 length=#        Screen length, component line limit
  85 offset=#        Positions to indent text
  86 overflowtext=t  Text to use at the beginning of an overflow line
  87 overflowoffset=# Positions to indent overflow lines
  88 compwidth=#     Positions to indent component text after first line
  89 nocomponent     Don't output "component: " for this component
  90 uppercase       Output text of this component in all upper case
  91 center          Center component on line (works for one-line components
  92                 only)
  93 clearscreen     Clear the screen (form feed) prior to each page
  94 leftadjust      Strip off leading spaces & tabs on each line of text
  95 compress        Change newlines in text to spaces
  96
  97 Where "=#" indicates a number must be specified, and "=t" indicates that
  98 arbitrary  text  up  to  end  of line or "," is required.  The variables
  99 without arguments are ON indicators, with the default in all cases  OFF.
 100 The  variables  "nocomponent",  center,  leftadjust and compress have no
 101 affect globally, and clearscreen only affects the global environment.
 102
 103 A line of the form:
 104
 105 ignores=component,...
 106
 107 specifies a list of components which are never output.
 108
 109 The component "MessageName" (case is unimportant) will output the actual
 110 message  name  (file  name) preceded by "folder:" if one is specified or
 111 found in the environment.
 112
 113 The component "Extras" will output all of the components of the  message
 114 which were not matched by explicit components, or included in the ignore
 115 list.  If this component is not specified, an ignore list is not  needed
 116 since all non-specified components will be ignored.
 117
 118 If "nocomponent" is NOT specified, then the component name will be output
 119 as it appears in the format file.
 120
 121 The current default format is:
 122
 123 width=80,length=40,overflowtext=***,overflowoffset=5
 124 ignores=msgid,message-id
 125 Date:leftadjust,offset=40
 126 To:leftadjust
 127 Cc:leftadjust
 128 :
 129 From:leftadjust
 130 Subject:leftadjust
 131 :
 132 extras:leftadjust,nocomponent
 133 :
 134 body:nocomponent
 135
 136
 137 Future work:(?)
 138
 139  o Fancier address formatting (to, cc, from)
 140  o Tests for "am i in this address list?" and what to do if yes/no
 141  o Currently only one component per line--should be generalized
 142  o Clear text and components intermixed on a line
 143  o Intelligent fill/justify per component?
 144
 145
 146
 147 May 1980
 148 Bruce