gemp: A Recursive CLI Expander of Go Template Files

Gemp reads pairs specifying Key-to-list-of-Value mappings K=V1,V2...Vn, and stores them for use according to the purpose of a specific subcommand gen or dump.

Typical use cases are generation of source code variants differing only by simple substitution, but possibly many combinations of values. Substitutions are performed by Go's text/template or fmt standard library packages.

For trivial examples, read on.

gen

Push Key-Value Pair Combinations through Source Code Template

Expand some template file according to all possible combinations of K=V1,V2... pairs, write into multiple output files, naming each according to its specific combination of values. Format of the template file is as specified in a Go standard library template file. Usable functionality from template is constrained by gemp's limitation that the data structure presented to the template.Execute() is simply a list of Key=Value pairs, both Key and Value of string type.

     # Write out a file containing valid ```template``` syntax
     $ echo '! Generated for {{.Color}} at {{.TimeHMS}}' >stamp+Color+.sh
     # Expand into two output files containing substitutions.
     $ gemp -format '-%.0s%s' Color="Blue,Red" TimeHMS="$(date +%H:%M:%S)," \
           gen -inkeyseparator '+' -outtopdir . stamp+Color+.sh
     $ more stamp-*.sh
     ::::::::::::::
     stamp-Blue.sh
     ::::::::::::::
     ! Generated for Blue at 21:04:42
     ::::::::::::::
     stamp-Red.sh
     ::::::::::::::
     ! Generated for Red at 21:04:42

In the example above, a call is made to Execute() for each of Blue and Red from the comma-separated list Color.

dump

Matching Constant Definitions Across Languages

Format all Key=Value pairs as specified on command line and write to standard output. A pattern in the format of Go's fmt package takes the place of the template file.

     # Store Key=Value pairs, separated by a newline character, into a shell variable
     $ KV=User=$USER\ TimeHMS="$(date +%H:%M:%S)"
     # Expand a JavaScript-specific format arg
     $ gemp -format 'export const %s = "%s";'$'' $KV dump
     export const User = "dmullis";
     export const TimeHMS = "20:41:06";
     # Expand a Go-specific format arg
     $ gemp -format 'const %s = "%s"'$'' $KV dump
     const User = "dmullis"
     const TimeHMS = "20:41:06"

Usage

Generic to both gen and dump commands.

Specific to gen.

If generating program source code, two difficulties may appear:

1. For a satisfactory experience when debugging stack traces, template expansions must match the number of lines in the template source code. Workaround examples may be found in _test_src/.
2. gofmt is confused by template syntax e.g. "{{...}}".

"gemp" is a portmanteau of "Go-tEMPlate".

See also

Other Go-based templating utilities, targeting somewhat different use cases:

• stringer
• Kubernetes templates
• gotpl