Moved over files from vieter repo

.gitignore

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+*.so

README.md

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+# Vconf
+
+Vconf is a small V library for loading in configuration from a TOML config file
+and/or environment variables.

v.mod

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+Module {
+	name: 'vconf'
+	description: 'A small library for reading configuration from a TOML file & environment variables'
+	version: '0.0.0'
+	license: 'MIT'
+	dependencies: []
+}

vconf.v

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+module vconf
+
+import os
+import toml
+
+// get_env_var tries to read the contents of the given environment variable. It
+// looks for either `${env.prefix}${field_name.to_upper()}` or
+// `${env.prefix}${field_name.to_upper()}${env.file_suffix}`, returning the
+// contents of the file instead if the latter. If both or neither exist, the
+// function returns an error.
+fn get_env_var(prefix string, field_name string, file_suffix string) ?string {
+	env_var_name := '$prefix$field_name.to_upper()'
+	env_file_name := '$prefix$field_name.to_upper()$file_suffix'
+	env_var := os.getenv(env_var_name)
+	env_file := os.getenv(env_file_name)
+
+	// If both are missing, we return an empty string
+	if env_var == '' && env_file == '' {
+		return ''
+	}
+
+	// If they're both set, we report a conflict
+	if env_var != '' && env_file != '' {
+		return error('Only one of $env_var_name or $env_file_name can be defined.')
+	}
+
+	// If it's the env var itself, we return it.
+	// I'm pretty sure this also prevents variable ending in _FILE (e.g.
+	// VIETER_LOG_FILE) from being mistakingely read as an _FILE suffixed env
+	// var.
+	if env_var != '' {
+		return env_var
+	}
+
+	// Otherwise, we process the file
+	return os.read_file(env_file) or {
+		error('Failed to read file defined in $env_file_name: ${err.msg()}.')
+	}
+}
+
+[params]
+struct LoadConfig {
+    prefix string
+    file_suffix string = '_FILE'
+    default_path string
+}
+
+// load<T> attempts to create an object of type T from the given path to a toml
+// file & environment variables. For each field, it will select either a value
+// given from an environment variable, a value defined in the config file or a
+// configured default if present, in that order.
+pub fn load<T>(conf LoadConfig) ?T {
+	mut res := T{}
+
+    // Later, this could be read from an env var as well.
+    path := conf.default_path
+
+	if os.exists(path) {
+		// We don't use reflect here because reflect also sets any fields not
+		// in the toml back to their zero value, which we don't want
+		doc := toml.parse_file(path)?
+
+		$for field in T.fields {
+			s := doc.value(field.name)
+
+			if s !is toml.Null {
+				$if field.typ is string {
+					res.$(field.name) = s.string()
+				} $else $if field.typ is int {
+					res.$(field.name) = s.int()
+				}
+			}
+		}
+	}
+
+	$for field in T.fields {
+		env_value := get_env_var(conf.prefix, field.name, conf.file_suffix)?
+
+		// The value of an env var will always take precedence over the toml
+		// file.
+		if env_value != '' {
+			$if field.typ is string {
+				res.$(field.name) = env_value
+			} $else $if field.typ is int {
+				res.$(field.name) = env_value.int()
+			}
+		}
+
+		// Now, we check whether a value is present. If there isn't, that means
+		// it isn't in the config file, nor is there a default or an env var.
+		mut has_value := false
+
+		$if field.typ is string {
+			has_value = res.$(field.name) != ''
+		} $else $if field.typ is int {
+			has_value = res.$(field.name) != 0
+		}
+
+		if !has_value {
+			return error("Missing config variable '$field.name' with no provided default. Either add it to the config file or provide it using an environment variable.")
+		}
+	}
+	return res
+}