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Better scripting

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Michael Snoyman 2017-01-03 16:56:48 +02:00
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posts/functors-applicatives-and-monads.md
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@ -1,16 +1,21 @@
__NOTE__ This content originally appeared on This content originally appeared on
[School of [School of
Haskell](https://www.schoolofhaskell.com/user/snoyberg/general-haskell/basics/functors-applicative-functors-and-monads). Haskell](https://www.schoolofhaskell.com/user/snoyberg/general-haskell/basics/functors-applicative-functors-and-monads).
Thanks for Julie Moronuki for encouraging me to update/republish, and for all Thanks for Julie Moronuki for encouraging me to update/republish, and for all
of the edits/improvements. of the edits/improvements.
__NOTE__ Code snippets below can be run using the [Stack build
tool](https://haskell-lang.org/get-started), by saving to a file `Main.hs` and
running with `stack Main.hs`. More information is available in the [How to
Script with Stack tutorial](https://haskell-lang.org/tutorial/stack-script).
Let's start off with a very simple problem. We want to let a user input his/her Let's start off with a very simple problem. We want to let a user input his/her
birth year, and tell him/her his/her age in the year 2020. birth year, and tell him/her his/her age in the year 2020.
Using the function `read`, this is really simple: Using the function `read`, this is really simple:
```haskell ```haskell
#!/usr/bin/env stack #!/usr/bin/env stack
-- stack --resolver lts-7.14 runghc -- stack --resolver lts-7.14 --install-ghc runghc
main = do main = do
putStrLn "Please enter your birth year" putStrLn "Please enter your birth year"
year <- getLine year <- getLine
@ -38,7 +43,7 @@ the following code:
```haskell ```haskell
#!/usr/bin/env stack #!/usr/bin/env stack
-- stack --resolver lts-7.14 runghc -- stack --resolver lts-7.14 --install-ghc runghc
import Text.Read (readMaybe) import Text.Read (readMaybe)
main = do main = do
@ -56,7 +61,7 @@ One way to do this is with pattern matching:
```haskell ```haskell
#!/usr/bin/env stack #!/usr/bin/env stack
-- stack --resolver lts-7.14 runghc -- stack --resolver lts-7.14 --install-ghc runghc
import Text.Read (readMaybe) import Text.Read (readMaybe)
main = do main = do
@ -75,7 +80,7 @@ calculating the age.
```haskell ```haskell
#!/usr/bin/env stack #!/usr/bin/env stack
-- stack --resolver lts-7.14 runghc -- stack --resolver lts-7.14 --install-ghc runghc
import Text.Read (readMaybe) import Text.Read (readMaybe)
displayAge maybeAge = displayAge maybeAge =
@ -112,7 +117,7 @@ our code with:
```haskell ```haskell
#!/usr/bin/env stack #!/usr/bin/env stack
-- stack --resolver lts-7.14 runghc -- stack --resolver lts-7.14 --install-ghc runghc
import Text.Read (readMaybe) import Text.Read (readMaybe)
displayAge maybeAge = displayAge maybeAge =
@ -149,7 +154,7 @@ we can change our code to not use `fmap`:
```haskell ```haskell
#!/usr/bin/env stack #!/usr/bin/env stack
-- stack --resolver lts-7.14 runghc -- stack --resolver lts-7.14 --install-ghc runghc
import Text.Read (readMaybe) import Text.Read (readMaybe)
displayAge maybeAge = displayAge maybeAge =
@ -189,7 +194,7 @@ with a simple implementation using pattern matching and then move back to `do`-n
```haskell ```haskell
#!/usr/bin/env stack #!/usr/bin/env stack
-- stack --resolver lts-7.14 runghc -- stack --resolver lts-7.14 --install-ghc runghc
import Text.Read (readMaybe) import Text.Read (readMaybe)
displayAge maybeAge = displayAge maybeAge =
@ -216,7 +221,7 @@ OK, it gets the job done... but it's very tedious. Fortunately, `do`-notation ma
```haskell ```haskell
#!/usr/bin/env stack #!/usr/bin/env stack
-- stack --resolver lts-7.14 runghc -- stack --resolver lts-7.14 --install-ghc runghc
import Text.Read (readMaybe) import Text.Read (readMaybe)
displayAge maybeAge = displayAge maybeAge =
@ -302,7 +307,7 @@ birthYearString` of type `Maybe Integer`. We can do this with `do`-notation:
```haskell ```haskell
#!/usr/bin/env stack #!/usr/bin/env stack
-- stack --resolver lts-7.14 runghc -- stack --resolver lts-7.14 --install-ghc runghc
import Text.Read (readMaybe) import Text.Read (readMaybe)
displayAge maybeAge = displayAge maybeAge =
@ -328,7 +333,7 @@ We can even use `fmap` twice and avoid the second slurp:
```haskell ```haskell
#!/usr/bin/env stack #!/usr/bin/env stack
-- stack --resolver lts-7.14 runghc -- stack --resolver lts-7.14 --install-ghc runghc
import Text.Read (readMaybe) import Text.Read (readMaybe)
displayAge maybeAge = displayAge maybeAge =
@ -361,7 +366,7 @@ see how it works in our example:
```haskell ```haskell
#!/usr/bin/env stack #!/usr/bin/env stack
-- stack --resolver lts-7.14 runghc -- stack --resolver lts-7.14 --install-ghc runghc
import Text.Read (readMaybe) import Text.Read (readMaybe)
displayAge maybeAge = displayAge maybeAge =
@ -411,7 +416,7 @@ so we'll automatically fix it by reversing the arguments to `yearDiff`. With
```haskell ```haskell
#!/usr/bin/env stack #!/usr/bin/env stack
-- stack --resolver lts-7.14 runghc -- stack --resolver lts-7.14 --install-ghc runghc
import Text.Read (readMaybe) import Text.Read (readMaybe)
displayAge maybeAge = displayAge maybeAge =
@ -442,7 +447,7 @@ main = do
```haskell ```haskell
#!/usr/bin/env stack #!/usr/bin/env stack
-- stack --resolver lts-7.14 runghc -- stack --resolver lts-7.14 --install-ghc runghc
import Control.Applicative ((<*>), Applicative) import Control.Applicative ((<*>), Applicative)
import Prelude (return, Monad) import Prelude (return, Monad)
import qualified Prelude import qualified Prelude
@ -481,7 +486,7 @@ main = do
```haskell ```haskell
#!/usr/bin/env stack #!/usr/bin/env stack
-- stack --resolver lts-7.14 runghc -- stack --resolver lts-7.14 --install-ghc runghc
returnMaybe = FIXME returnMaybe = FIXME
main main
@ -515,7 +520,7 @@ main = do
```haskell ```haskell
#!/usr/bin/env stack #!/usr/bin/env stack
-- stack --resolver lts-7.14 runghc -- stack --resolver lts-7.14 --install-ghc runghc
import Text.Read (readMaybe) import Text.Read (readMaybe)
displayAge maybeAge = displayAge maybeAge =
@ -566,7 +571,7 @@ main = do
```haskell ```haskell
#!/usr/bin/env stack #!/usr/bin/env stack
-- stack --resolver lts-7.14 runghc -- stack --resolver lts-7.14 --install-ghc runghc
import Text.Read (readMaybe) import Text.Read (readMaybe)
import Control.Applicative ((<$>), (<*>)) import Control.Applicative ((<$>), (<*>))
@ -607,7 +612,7 @@ main = do
```haskell ```haskell
#!/usr/bin/env stack #!/usr/bin/env stack
-- stack --resolver lts-7.14 runghc -- stack --resolver lts-7.14 --install-ghc runghc
import Text.Read (readMaybe) import Text.Read (readMaybe)
import Control.Applicative ((<$>), (<*>)) import Control.Applicative ((<$>), (<*>))