This library provides a simple solution to a common use case in configuration variables:

A delimiter is needed to accompany optional data, but must not be present when the optional data is missing.

Traditionally this scenario is solved in one of two ways:

• Treat the delimiter as required, forcing a default value to always be defined and available.
• Embed the optional delimiter in the actual value of the optional data, essentially corrupting the value of the data (PORT="80" becomes PORT=":80" but clearly the value :80 is not a valid port number.)

Don't expect to use templ without writing a little code around it: it is primitive. This library Does One Thing: the substitution processing. You'll need to provide the bindings into your context and use case.

The solution pattern provided by templ is to include the delimiter(s) inside the double-braces template substitution syntax {{}} (as a prefix and/or a suffix).

API_HOST="api.example.com"
API_ENDPOINT="{{API_HOST}}{{:API_PORT}}"

(With API_PORT undefined, the : is excluded from the result.)

API_HOST="api.example.com"
API_PORT="8080"
API_ENDPOINT="{{API_HOST}}{{:API_PORT}}"

(With API_PORT defined, the : is included in the result.)

There are eighteen supported delimiters:

1. <space>
2. ,
3. .
4. ;
5. :
6. ?
7. &
8. @
9. #
10. /
11. (
12. )
13. <
14. >
15. _
16. -
17. \
18. |

• Delimiters can be used as a prefix and/or suffix.
• Any number of delimiters can be used in any combination.

Variables can be dynamic.

INSTANCE="3"
HOST_1="devhost1.example.com"
HOST_2="devhost2.example.com"
HOST_3="qa.example.com"
HOST_4="example.com"
SERVICE_ENDPOINT="{{HOST_{{INSTANCE}}}}"

This usefulness is compounded when multiple variables are resolved:

INSTANCE="3"
HOST_1="devhost.example.com"
PORT_1="8080"
HOST_2="devhost.example.com"
PORT_2="8081"
HOST_3="qa.example.com"
HOST_4="example.com"
SERVICE_ENDPOINT="{{HOST_{{INSTANCE}}}}{{:PORT_{{INSTANCE}}}}"

The value returned by a template evaluation can be converted to uppercase using the ^ character before the variable name.

ENV="dev"
HOST_DEV="vm123.example.com"
HOST_QA="qa.example.com"
HOST_PROD="example.com"
SERVICE_ENDPOINT="{{HOST_{{^ENV}}}}"

This example shows both the ^ UPPERCASE syntax, and the nested-template resolution to resolve values dynamically.