Conditionally running IO actions based on environment variables.

This package provides functions for validating presence / contents of environment variables, and conditionally running IO actions based on the result.

Introduction

This package is used to guard IO actions behind an environment variable.

Examples

Simple

guardSet :: String -> IO a -> IO (Maybe a)

-- Run function if env var AppEnv is set (to anything).
guardSet "AppEnv" runProd

guardEquals :: String -> String -> IO a -> IO (Maybe a)

-- Run function if env var AppEnv is set to "prod" (case-insensitive).
guardEquals "AppEnv" "prod" runProd

guardPredicate :: String -> (String -> Bool) -> IO a -> IO (Maybe a)

-- Run function if env var AppEnv's value satisfies the predicate.
guardPredicate "AppEnv" (\s -> s == "staging" || s == "dev") runTests

Higher-Level Combinators

-- withGuard uses the 'ExpectEnv' type to determine which of the above three
-- functions to call.
withGuard :: String -> ExpectEnv -> IO a -> IO (Maybe a)

-- equivalent to 'guardSet "BUILD_DOCS" buildDocs'.
withGuard "BUILD_DOCS" ExpectEnvSet buildDocs

-- equivalent to 'guardEquals "BUILD_DOCS" "true" buildDocs'.
withGuard "BUILD_DOCS" (ExpectEnvEquals "true") buildDocs

-- guardOrElse runs one of two IO actions, depending on the success of the env
-- variable lookup.
guardOrElse :: String -> ExpectEnv -> IO a -> IO e -> IO (Either e a)

-- Runs runA if DO_A is set; otherwise runs runB.
guardOrElse "DO_A" ExpectEnvSet runA runB

-- guardOrElse' specialized to the same type.
guardOrElse' :: String -> ExpectEnv -> IO a -> IO a -> IO a

-- Runs runTests if RUN_TESTS is set; otherwise prints a message.
guardOrElse' "RUN_TESTS" ExpectEnvSet runTests (putStrLn "*** Tests Disabled ***")