Updates Readme

README.md

@@ -2,18 +2,53 @@
 
 [![kriti-lang::CI](https://github.com/hasura/kriti-lang/actions/workflows/haskell.yml/badge.svg)](https://github.com/hasura/kriti-lang/actions/workflows/haskell.yml)
 
-A minimal json templating language based on Go' template language.
-
-## TODO
-- Improve and extend lexer errors
-- Pretty printer
-- Improve span generation
-- Array membership parsing
-- CLI interface
-- List flatten operation
-
-## Uses
-This library exposes the function `runKriti`, the type definition of the function is:
+A minimal json templating language inspired by Go's template language.
+
+Kriti templates are a superset if JSON with path lookups, if/the/else expressions, loops, and some basic predicate and conditional operators.
+
+Kriti expressions are wrapped in double curly brackets such as `"http://wwww.google.com/{{$body.path}}"`. The `Kriti` evaluator takes `Kriti` template and a set of source json expressions paired with binders then uses them to construct a new json expression.
+
+### Kriti Expressions
+
+#### Path Accessors
+
+Values can be looked up in bound json expressions using the standard path lookup syntax:
+```
+{{ $.foo.bar[0] }}
+```
+`.` is used to look up object fields and `[x]` is used to lookup array indices.
+
+#### Loops
+
+The `range` identifier is used to declare for loops:
+```
+{{ range i, x := $.event.author.articles }}
+  {{ x.title }}
+{{ end }}
+```
+`i` and `x` above are binders for the index and value of the array element from `$.event.author.articles`. The index can be omitted by using an underscore in its place.
+
+#### If Statements
+Kriti supports if statements and `>` `<` `==` `||` and `&&` operators.
+```
+{{ if x.published && (x.post_id > 100) }}
+    {
+      "id": {{x.id}},
+      "title": {{x.title}}
+    }
+{{ else }}
+    null
+{{ end }}
+```
+
+#### String Interpolation
+String Interpolation is currently limited to path lookups:
+```
+"http://www.{{$.domain}}.com/{{$.path}}"
+```
+
+### Library
+The library exposes the function `runKriti`, the type definition of the function is:
 ``` haskell
 runKriti :: Text -> [(Text, Value)] -> Either KritiErr Value
 ```
@@ -22,17 +57,13 @@ The first argument of the function is the template JSON, for example, we can use
 myTemplate :: Text
 myTemplate =
     "{\
-    \   'name': {{x.name.english}},\
-    \   'id': {{x.id}},\
-    \   'hp': {{x.base.HP}}\
+    \   'name': {{$.name.english}},\
+    \   'id': {{$.id}},\
+    \   'hp': {{$.base.HP}}\
     \}"
 ```
 
-The second argument is a `list` of `tuple` of `(Text, Value)`. The first element of the tuple is the binding to be used for the JSON object, i.e. for the above template we are using `x` as the JSON binding, so, `x` will bind to the JSON object. The second element of the tuple is of type `Data.Aeson.Value` (can be obtained by `Data.Aeson.decode` method).
-
-The function `runKriti` will return `Either KritiErr Value`. If the parser is successful, then it will return `Right Value`, else it will return `Left KritiErr` which can be used for debugging.
-
-## Run example
+#### Library Usage Sample Program
 To run the example, first clone this repository using the following command:
 ``` sh
 git clone git@github.com:hasura/kriti-lang.git
@@ -42,7 +73,20 @@ Now, run the following command:
 cd kriti-lang
 cabal new-run example
 ```
-## Examples
+
+The second argument is a `list` of `tuple` of `(Text, Value)`. The first element of the tuple is the binding to be used for the JSON object, i.e. for the above template we are using `x` as the JSON binding, so, `x` will bind to the JSON object. The second element of the tuple is of type `Data.Aeson.Value` (can be obtained by `Data.Aeson.decode` method).
+
+The function `runKriti` will return `Either KritiErr Value`. If the parser is successful, then it will return `Right Value`, else it will return `Left KritiErr` which can be used for debugging.
+
+### CLI Tool
+The executable is a CLI tool which applices a transformation to a single json file:
+``` bash
+➜ cabal run kriti -- --json test/data/eval/success/source.json --template test/data/eval/success/examples/example1.kriti
+{"guid":"43a922da-9665-4099-8dfc-f9af369695a4"}
+```
+The binder for the source file can be changed wit the `--bind` flag.
+
+## Transformation Examples
 
 JSON Input:
 ```