Merge branch 'cufp'
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Gabriel Gonzalez
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slides/slides.md
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slides/slides.md
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% Haskell for Shell Scripting
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% Gabriel Gonzalez
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% April 8, 2015
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% September 3, 2015
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# Before class
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If you haven't installed `ghc`, yet:
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If you haven't installed `ghc`, visit:
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```bash
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$ echo "/home/ggonzalez/tools/ghc-7.8.3-Darwin.x86_64" >> ~/.tools
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$ sync-dottools.sh
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```
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* [https://www.haskell.org/downloads](https://www.haskell.org/downloads)
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... then open a new terminal window.
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... and install a GHC distribution appropriate to your operating system.
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To test your Haskell installation, run these commands:
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To test your Haskell installation, run these commands from a terminal:
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```bash
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$ echo 'main = putStrLn "Hello, world!"' > hello.hs
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@ -25,7 +22,7 @@ Install the shell scripting library using these commands:
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```bash
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$ cabal update
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$ cabal install turtle-1.1.0
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$ cabal install turtle-1.2.1
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```
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# Outline
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@ -41,7 +38,7 @@ $ cabal install turtle-1.1.0
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* Folds
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* Patterns
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I've hosted slides on go/learn so that people can follow along locally
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I'm hosting slides on Github so that people can follow along locally
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# Overview of Haskell
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@ -58,13 +55,13 @@ Haskell can be both **interpreted** or **compiled** to a native binary
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Haskell is a managed language, providing garbage collection, concurrency, and
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transactional shared memory:
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* **Garbage collection** is generational and efficient (measured in GB / s)
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* **Concurrency** uses green-threads and is efficient (world record for SDN)
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* **Transactional memory** simplifies race-free concurrent code without polling
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* **Garbage collection** is efficient (throughput measured in GB / s)
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* **Concurrency** uses green-threads and is efficient (can handle 1M threads)
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* **Transactional memory** simplifies race-free concurrent code
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# Biggest disadvantages of Haskell
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# Big disadvantages of Haskell
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* Not a JVM language
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* No JVM backend
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* Beginners can't easily reason about performance
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* Built-in record syntax is clumsy
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* Most language features are libraries, which hampers discoverability
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@ -122,7 +119,7 @@ import Turtle --
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main = echo "Hello, world!" -- echo Hello, world!
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```
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... then run the example:
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... then run the example script:
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```bash
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$ chmod u+x example.hs
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@ -133,7 +130,7 @@ Hello, world!
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# Create a native binary
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```bash
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$ ghc -O2 -threaded example.hs
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$ ghc -O2 example.hs
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$ ./example
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Hello, world!
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```
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@ -253,6 +250,8 @@ The top level of a Haskell program is declarative and only allows definitions
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You cannot execute code at the top level
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The runtime only executes `main`!
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# Subroutines
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Use `do` to create a subroutine that runs more than one command:
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@ -312,7 +311,7 @@ main = do --
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```haskell
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$ ./example.hs
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2015-01-24 03:40:31 UTC
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2015-09-01 23:56:03.245 UTC
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```
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Why not this?
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@ -341,6 +340,17 @@ scala> for { x <- Seq(1, 2); y <- Seq(3, 4) } yield (x, y)
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res0: Seq[(Int, Int)] = List((1,3), (1,4), (2,3), (2,4))
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```
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... or LINQ/`from`/`select` in C#:
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```cs
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List<int> xs = new List<int> { 1, 2 }
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List<int> ys = new List<int> { 3, 4 }
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var result =
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from x in xs
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from y in ys
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select Tuple<int, int>(x, y)
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```
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# Nesting subroutines
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```haskell
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@ -362,7 +372,7 @@ Same result:
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```haskell
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$ ./example.hs
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2015-01-24 03:40:31 UTC
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2015-09-01 23:56:03.245 UTC
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```
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# Unnecessary `return`
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@ -481,7 +491,6 @@ main = do
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```haskell
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$ ghci -v0
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Prelude> :set -XOverloadedStrings
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Prelude> import Turtle
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```
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@ -909,16 +918,12 @@ format (s%" failed with exit code: "%d) :: Text -> Int -> Text
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# Exercise
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What do you think these print out?
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What do you think this prints out?
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```haskell
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Prelude Turtle> format ("A "%s%" string that takes "%d%" arguments") "format" 2
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```
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```haskell
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Prelude Turtle> format "I take 0 arguments"
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```
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# The `Format` type
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A format string is not `Text`!
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@ -932,6 +937,7 @@ So what is going on here?
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```haskell
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Prelude Turtle> format "I take 0 arguments"
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"I take 0 arguments"
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```
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# `Format` implements `IsString`
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```haskell
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(%) :: Format b c -> Format a b -> Format a c
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"A " :: Format a a
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s :: Format a (String -> a)
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" string that takes " :: Format a a
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d :: Format a (Int -> a)
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" arguments" :: Format a a
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"A " :: Format a a
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s :: Format a (Text -> a)
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" string that takes " :: Format a a
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d :: Format a (Int -> a)
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" arguments" :: Format a a
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"A "%s%" string that takes "%d%" arguments" :: Format a (Text -> Int -> a)
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format "A "%s%" string that takes "%d%" arguments" :: Text -> Int -> Text
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format ("A "%s%" string that takes "%d%" arguments") :: Text -> Int -> Text
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```
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You can build your own format specifiers!
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@ -1487,7 +1493,7 @@ Fold.head :: Fold a (Maybe a)
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```
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```haskell
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ls :: Shell Turtle.FilePath
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ls "/tmp" :: Shell Turtle.FilePath
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fold :: Shell a -> Fold a b -> IO b
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fold (ls "/tmp") :: Fold Turtle.FilePath b -> IO b
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# Questions?
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# Backup utility example
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```haskell
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{-# LANGUAGE OverloadedStrings #-}
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import Turtle
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import Prelude hiding (FilePath)
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parser = (,) <$> argPath "src" "Source directory"
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<*> argPath "dst" "Destination directory"
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backup file = do
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exists <- testfile file
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when exists (do
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let backupFile = file <.> "bak"
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backup backupFile
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mv file backupFile )
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main = do
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(src, dest) <- options "Backup a directory" parser
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sh (do
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inFile <- lstree src
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Just suffix <- return (stripPrefix src inFile)
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let outFile = dest </> suffix
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backup outFile
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echo (format ("Copying "%fp%" to "%fp) inFile outFile)
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cp inFile outFile )
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echo "Done!"
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```
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# Command line usage
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```bash
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$ ./backup --help
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Backup a directory
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Usage: backup SRC DST
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Available options:
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-h,--help Show this help text
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SRC Source directory
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DST Destination directory
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```
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```bash
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$ ./backup a/ b/
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Copying a/1 to b/1
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Copying a/2 to b/2
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$ ls b/
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1 2
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$ ./backup a/ b/
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Copying a/1 to b/1
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Copying a/2 to b/2
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$ ls b/
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1 1.bak 2 2.bak
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```
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# Conclusions
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You can use Haskell as a "better Bash", getting types for free without slow
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startup times or heavyweight syntax.
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If you want others to run your Haskell scripts, they can use `dottools` to install
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`ghc` on their machine.
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I also have a relocatable `ghc` uploaded to Packer that you can use to interpret
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scripts on Mesos.
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We also have an internal Hackage server at Twitter (go/hackage)
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Visit https://hackage.haskell.org/package/turtle for more extensive documentation
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on the shell scripting library we used today
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