dater

A simple DSL for calculating dates, with a toplevel interpreter implemented in Haskell.

No, unfortunately, this is not about dating.

The Language

Formal grammar

S     ::= DATE | DATE NUMOP YMD | DATE DIFF DATE
NUMOP ::= + | -
DIFF  ::= <>
DATE  ::= YMD | TODAY

Description

A dater program consists of just one command: adding dates or finding the difference two dates. Each date consists of a year, month, and a day.

The add operator

The add + operator takes a y/m/d date and a y/m/d date shift to produce a new date.

dater $> 2000/3/2 + 1/0/3
2001/3/5

The subtraction operator

The add - operator takes a y/m/d date and a y/m/d date shift to produce a new date. When today is 2022/11/19:

dater $> today - 0/22/33
2020/12/17

The difference operator

The difference <> operator will find the distance between two dates.

dater $> 2021/11/12 <> 2022/1/1
0/1/20

Syntactic Sugar

Five syntactic sugar functions exist to make the dater easier to use.

* day   x  => 0 0 x
* week  y  => 0 0 7y
* month z  => 0 z 0
* year  w  => w 0 0
* today    => today's date

Example:

dater $> 2012/12/22 + week 10
2013/3/2

Usage

The recommended way to use the language is with the toplevel interpreter in this repository.

Installation

First, you will need The Haskell Stack. Then, simply run

stack install

This will copy the dater binary to your path (~/.local/bin/ on Linux systems).

Running the toplevel

➜  ~ dater
Welcome to dater, date calculations done easy!
Example Usages:
dater $> today +  week 9
dater $> 2022/11/22 - 0/11/1
dater $> today <> 1997 / 4 / 10

dater $>

When you are done, exit with

dater $>:q
Goodbye.

Add-list

More features!

• Use previous computation value (Python's _ or ghci's it)
• Run files instead of running the REPL, e.g. dater foo.dtr
• Option to specify as days: as_days today <> 1997/4/10. This could be a toggle option.

Implementation details

• Better mtl instead of IO (Either DateError a)
• Migrate Main.hs out of InputT