# Effects utilities

Contains utility functions extending the functionality of the `eff` package.

```idris
module Util.Eff

import Control.Eff
import Text.ANSI

import System
import System.File

%default total
```

## Logging

### Log Levels

Basic enumeration describing log levels, we define some (hidden) utility functions for working with these.

```idris
public export
data Level = Err | Warn | Info | Debug | Trace | Other Nat 
```

<!-- idris
export
levelToNat : Level -> Nat
levelToNat Err = 0
levelToNat Warn = 1
levelToNat Info = 2
levelToNat Debug = 3
levelToNat Trace = 4
levelToNat (Other k) = k

export
natToLevel : Nat -> Level
natToLevel 0 = Err
natToLevel 1 = Warn
natToLevel 2 = Info
natToLevel 3 = Debug
natToLevel 4 = Trace
natToLevel k = Other k

export
Eq Level where
  x == y = levelToNat x == levelToNat y

export
Ord Level where
  compare x y = compare (levelToNat x) (levelToNat y)

export infixl 8 >+
export
(>+) : Level -> Nat -> Level
(>+) x k = natToLevel (k + levelToNat x)
-->

Convert a `Level` into a colorized tag

```idris
export
levelToTag : Level -> String
levelToTag Err =
    show . bolden . show . colored BrightRed $ "[Error]"
levelToTag Warn =
    show . bolden . show . colored BrightYellow $ "[Warning]"
levelToTag Info =
    show . bolden . show . colored Green $ "[Info]"
levelToTag Debug =
    show . bolden . show . colored Magenta $ "[Debug]"
levelToTag Trace =
    show . bolden . show . colored Cyan $ "[Trace]"
levelToTag (Other k) =
    show . bolden . show . colored BrightWhite $ "[\{show k}]"
```

### Logger effect

This is a basic data structure that captures a lazy log message (so we don't have to pay any of the costs associated with generating the log message when it is filtered)

```idris
public export
data Logger : Type -> Type where
  Log : (level : Level) -> (msg : Lazy String) -> Logger ()
```

We'll also provide some basic accessors, and an `ignore` function useful for writing handlers.

```idris
export  
(.level) : Logger t -> Level
(.level) (Log level msg) = level

export
(.msg) : Logger t -> Lazy String
(.msg) (Log level msg) = msg

export
ignore : Logger t -> t
ignore (Log level msg) = ()
```

#### Handler

Because we know that we will only be using `logger` in an `IO` context, we aren't currently going to provide a `runLogger` or the like, instead we'll define a function, suitable for use as a `runEff` handler, that filters log messages and prints them to `stderr` over `IO`.

In the event a log message is filtered out, it's inner message is never inspected, avoiding evaluation.

```idris
export
handleLoggerIO : 
  (max_level : Level) -> Logger t -> IO t
handleLoggerIO max_level x = 
  if x.level <= max_level 
    then do
      _ <- fPutStrLn stderr "\{levelToTag x.level}: \{x.msg}"
      pure . ignore $ x
    else pure . ignore $ x
```

Use the `WriterL "log" String` effect like a logging library. We'll provide a few "log levels" as verbs for the effect, but no filtering is done, when logging is enabled, all logs are always displayed, however the log level is indicated with a colored tag. 

```idris
export
error : Has Logger fs => Lazy String -> Eff fs ()
error x = send $ Log Err x

export
warn : Has Logger fs => Lazy String -> Eff fs ()
warn x = send $ Log Warn x

export
info : Has Logger fs => Lazy String -> Eff fs ()
info x = send $ Log Info x

export
debug : Has Logger fs => Lazy String -> Eff fs ()
debug x = send $ Log Debug x

export
trace : Has Logger fs => Lazy String -> Eff fs ()
trace x = send $ Log Trace x
```