Plan how the program should work and add docs

README.md

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 # task-machine
+
+A TUI client for the [todo.txt](https://github.com/todotxt/todo.txt) format, written in Haskell,
+that supports automatically creating new tasks based on template tasks.
+
+For an introduction to the file format, see the [todo.txt readme](https://github.com/todotxt/todo.txt/blob/master/README.md).
+
+## Template tasks
+
+Template tasks allow for automatically creating new tasks on certain dates.
+
+They work by evaluating a formula for the current day.
+If the formula evaluates to `true`, a new task gets added to the current day using the template text specified.
+If it evaluates to `false`, nothing happens.
+
+When creating a template task, a start date can be specified.
+In that case, the formula will be evaluated for every day from the starting date to the current date.
+
+Once all tasks are added, the starting date will be set to one day after the current date automatically,
+to avoid evaluating any day twice, even if no starting date was specified initially.
+
+Template tasks are inspired by Ben Crowell's [when](https://github.com/bcrowell/when) calendar program.
+
+### Format
+
+This is the format for template tasks.
+The square brackets `[]` indicate an optional part of the syntax.
+The curly braces `{}` are part of the syntax.
+
+`# [start-date] {when-formula} [priority] template-text`
+
+- The `#` signals that this line is a template task, not a normal task.
+
+- *Optional*: All days from `start-date` to the current date are tested when creating new tasks.
+It should be in the `YYYY-MM-DD` format.
+If no `start-date` is specified, it is assumed to be the current date.
+
+- The `when-formula` is evaluated in order to decide whether a new task should be created for a day.
+
+- *Optional*: The `priority` is a regular todo.txt priority, for example `(E)`.
+
+- The `template-text` is pretty much a normal task description.
+The only difference is that it supports date expressions.
+
+#### Examples
+
+`# 2018-09-08 {day_of_week == fri} clean the kitchen @home`
+
+`# {d2018-10-20 - today >= 0} (A) daily every day up to (and including) 2018-10-20 with priority A`
+
+`# {day == 20 && month == 10} Tom's birthday ({year - 1978} years old)`
+(if Tom was born on 1978-10-20)
+
+### Date expression
+
+***TODO***: Document properly once it's actually implemented
+
+- *date* + - *number* → *date*
+- *date* - *date* → *number*
+- *number* + - \* / *number* → *number*
+- *number* == != > < >= <= *number* → *boolean*
+- *boolean* && || == != *boolean* → *boolean*
+
+### When formula
+
+### Template text

plan.txt

@@ -1,3 +1,76 @@
+Inspired by https://github.com/todotxt/todo.txt
+
+-----< Basic concept >-----
+The program loads and operates on one todo file at a time.
+A todo file is a text file where (with a few exceptions) every line is one todo item.
+
+-----< Functionality >-----
+- load file (parsing)
+- save file
+
+- prune completed tasks
+
+- "running tasks":
+Two-line structure that contains:
+1. A line to determine when, and set variables (both using calendar formulas)
+2. A "task recipe" that can use $variables (for birthdays etc.)
+Running tasks are computed in the order they appear in the file.
+
+-----< How it works >-----
+1. $ todo work.todo
+2. load the entire file
+	2.1 parse lines
+	2.2 preserve structuring of the file: empty lines, comments, order
+	2.3 if there's any errors, abort instead of possibly overwriting the file with wrong data later
+3. calculate any running tasks
+	3.1 look at the last-calculated date, current date and task duration
+	3.2 calculate new tasks and append them to the file
+	3.3 update last-calculated date of the task
+	3.4 save updates
+4. present user interface (TUI)
+	4.1 show tasks
+	4.2 search and order tasks
+	4.3 check tasks
+	4.4 Optional: delete tasks
+	4.5 Optional: create new tasks
+	4.5 Optional: edit existing tasks (modify text, priority, ...)
+5. overwrite file with new changes
+	5.1 instant mode
+		5.1.1 after initial running task calculation
+		5.1.2 whenever any task was changed (checked, edited, deleted, ...)
+	5.2 deferred mode
+		5.2.1 save on user save action
+		5.2.2 save on close
+	5.3 default mode: instant mode?
+
+-----< The file format >-----
+normal tasks:
+[x] [priority] [[completion-date] creation-date] text
+where completion-date can only be specified if the task was actually completed
+
+running tasks:
+# [running-date] when-formula template
+
+text:
+- may include any amount of +projects and @contexts
+	(+projects and @contexts are case insensitive)
+- may include key:value specifiers
+	(key and value must consist of non-whitespace characters which are not colons)
+	(keys are case insensitive too)
+
+template:
+- consists of any string
+- may include {expressions} which are when-formulas
+	(if the when-formula returns a date, the expression is replaced by its YYYY-MM-DD representation)
+	(if the when-formula returns a number, the expression is replaced by that number)
+	(if the when-formula returns a boolean, the expression is replaced by "yes" or "no")
+
+-----< Examples/Testing >-----
+The program should work with all https://github.com/todotxt/todo.txt compliant todo files.
+"Running tasks" are merely an extension of the todo format.
+They use a syntax similar to what I expect comments would look in todo.txt 2.0.
+In todo.txt compliant programs, they simply show up as not completed tasks.
+
 Starting the program
  - read command line options
  - export default config