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---
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language: phel
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filename: learnphel.phel
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contributors:
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- ["Chemaclass", "https://github.com/Chemaclass"]
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---
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[Phel](https://phel-lang.org/) is a functional programming language that compiles to PHP.
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It is a dialect of Lisp inspired by Phel and Janet.
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## Features
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- Built on PHP's ecosystem
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- Good error reporting
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- Persistent Datastructures (Lists, Vectors, Maps and Sets)
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- Macros
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- Recursive functions
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- Powerful but simple Syntax
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- REPL
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```phel
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# Comments begin with a # character and continue until the end of the line. There are no multi-line comments.
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# Phel is written in "forms", which are just
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# lists of things inside parentheses, separated by whitespace.
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# The first call in a file should be ns, to set the namespace
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(ns learn-phel)
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# More basic examples:
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# str will create a string out of all its arguments
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(str "Hello" " " "World") # => "Hello World"
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# Math is straightforward
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(+ 1 1) # => 2
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(- 2 1) # => 1
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(* 1 2) # => 2
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(/ 2 1) # => 2
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# Equality is =
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(= 1 1) # => true
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(= 2 1) # => false
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# You need not for logic, too
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(not true) # => false
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# Nesting forms works as you expect
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(+ 1 (- 3 2)) # = 1 + (3 - 2) => 2
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# Phel inherits PHP under the hood, so it can use native PHP (functions and classes) without any additional cost
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# by using the `php/` prefix to all PHP native functions.
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# Types
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#############
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# Booleans are similar as the native PHP ones
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nil
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true
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false
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# Symbols are used to name functions and variables in Phel
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# For example: symbol, snake_case_symbol, my-module/my-function
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# Keywords are like symbols that begin with a colon character. However, they are used as constants rather than a name for something.
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:keyword
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:0x0x0x
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::
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# Numbers in Phel are equivalent to numbers in PHP
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1337 # integer
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+1337 # positive integer
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-1337 # negative integer
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1.234 # float
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+1.234 # positive float
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-1.234 # negative float
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1.2e3 # float
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7E-10 # float
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# Strings are surrounded by double quotes. They almost work the same as PHP double quoted strings.
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# A string can be written in multiple lines. The line break character is then ignored by the reader.
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"hello world"
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"this is\na\nstring"
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"this
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is
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a
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string."
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"use backslack to escape \" string"
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"the dollar must not be escaped: $ or $abc just works"
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# Collections & Sequences
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#############
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# Lists are linked-list data structures, while vectors are array-backed.
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(type '(1 2 3)) # :list
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(type [1 2 3]) # :vector
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# A list would be written as just (1 2 3), but we have to quote
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# it to stop the reader thinking it's a function.
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# Also, (list 1 2 3) is the same as '(1 2 3)
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# You can produce a (non-lazy) sequence between a range.
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(range 1 10 2) # <- (range from to step)
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(take 4 (range 10))
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# Use cons to add an item to the beginning of a list
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(cons 4 '(1 2 3)) # => (4 1 2 3)
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# Use push to add, and put to replace an item in a vector
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(push [1 2 3] 4) # => (1 2 3 4)
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(put [1 2 3] 1 4) # => (1 4 3)
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# Use concat to add lists or vectors together
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(concat [1 2] '(3 4)) # => [1 2 3 4]
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# Use filter, map to interact with collections
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(map inc [1 2 3]) # => [2 3 4]
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(filter even? [1 2 3]) # => [2]
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# Use reduce to reduce them. The initial-value is mandatory
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(reduce + 0 [1 2 3 4])
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# => (+ (+ (+ 1 2) 3) 4)
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# => 10
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(reduce push [] '(3 2 1))
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# = (push (push (push [] 3) 2) 1)
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# => [3 2 1]
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# Functions
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#############
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# Use fn to create new functions. A function always returns its last statement.
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(fn [] "Hello World") # => :function
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# You need extra parens to call it
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((fn [] "Hello World")) # => "Hello World"
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# You can create a var using def
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(def x 1)
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x # => 1
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# Assign a function to a var
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(def hello-world (fn [] "Hello World"))
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(hello-world) # => "Hello World"
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# You can shorten this process by using defn
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(defn hello-world [] "Hello World")
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# The [] is the list of arguments for the function.
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(defn hello [name]
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(str "Hello " name))
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(hello "Jens") # => "Hello Jens"
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# You can also use this shorthand to create functions:
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(def hello2 |(str "Hello " $1))
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(hello2 "Anna") # => "Hello Anna"
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# Functions can pack extra arguments up in a seq for you
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(defn count-args [& args]
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(str "You passed " (count args) " args: " args))
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(count-args 1 2 3) # => "You passed 3 args: @[1 2 3]"
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# You can mix regular and packed arguments
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(defn hello-count [name & args]
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(str "Hello " name ", you passed " (count args) " extra args"))
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(hello-count "Jesus" 1 2) # => "Hello Jesus, you passed 2 extra args"
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# Maps
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#############
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# Hash maps have faster lookups but don't retain key order.
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(type {:a 1 :b 2 :c 3}) # => :hash-map
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(type (hash-map :a 1 :b 2 :c 3)) # => :hash-map
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# Maps can use any hashable type as a key, but usually keywords are best
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# Keywords are like strings with some efficiency bonuses and they start with `:`
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(type :a) # => :keyword
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(def stringmap {"a" 1 "b" 2 "c" 3})
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stringmap # => {"a" 1 "b" 2 "c" 3}
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(def keymap {:a 1 :b 2 :c 3})
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keymap # => {:a 1 :c 3 :b 2}
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# Retrieve a value from a map by calling it as a function
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(stringmap "a") # => 1
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(keymap :a) # => 1
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# Keywords can be used to retrieve their value from a map, too!
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(:b keymap) # => 2
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# Don't try this with strings.
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# ("a" stringmap)
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# ...Exception: Call to undefined function a()
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# Retrieving a non-present key returns nil
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(stringmap "d") # => nil
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# Use put to add new keys to hash-maps
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(def newkeymap (put keymap :d 4))
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newkeymap # => {:a 1 :b 2 :c 3 :d 4}
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# But remember, phel types are immutable!
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keymap # => {:a 1 :b 2 :c 3}
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# Use unset to remove keys
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(unset keymap :a) # => {:b 2 :c 3}
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# Sets
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#############
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# A Set contains unique values in random order.
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(type (set 1 2 3)) # => :set
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(set 1 2 3 1 2 3 3 2 1 3 2 1) # => (set 1 2 3)
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# Add a member with push
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(push (set 1 2 3) 4) # => (set 1 2 3 4)
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# Remove one with unset
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(unset (set 1 2 3) 1) # => (set 2 3)
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# Test for existence by using the set as a function:
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((set 1 2 3) 1) # => 1
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((set 1 2 3) 4) # => nil
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# There are more functions like: count, union, intersection, difference, etc.
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# Useful forms
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#############
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# Logic constructs in clojure are just macros, and look like everything else
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(if false "a" "b") # => "b"
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(if false "a") # => nil
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# Use let to create temporary bindings
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(let [a 1 b 2]
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(> a b)) # => false
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# Group statements together with do
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(do
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(print "Hello")
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"World") #=> "World" (prints "Hello")
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# Functions have an implicit do
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(defn print-and-say-hello [name]
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(print "Saying hello to " name)
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(str "Hello " name))
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(print-and-say-hello "Jeff") #=> "Hello Jeff" (prints "Saying hello to Jeff")
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# So does let
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(let [name "Urkel"]
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(print "Saying hello to " name)
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(str "Hello " name)) # => "Hello Urkel" (prints "Saying hello to Urkel")
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# Use the threading macros (-> and ->>) to express transformations of
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# data more clearly.
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# The "Thread-first" macro (->) inserts into each form the result of
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# the previous, as the first argument (second item)
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(->
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{:a 1 :b 2}
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(put :c 3) #=> (put {:a 1 :b 2} :c 3)
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(unset :b)) #=> (unset (put {:a 1 :b 2} :c 3) :b)
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# The double arrow does the same thing, but inserts the result of
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# each line at the *end* of the form. This is useful for collection
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# operations in particular:
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(->>
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(range 10)
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(map inc) #=> (map inc (range 10))
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(filter odd?)) #=> (filter odd? (map inc (range 10)))
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# Result: [1 3 5 7 9]
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# When you are in a situation where you want more freedom as where to
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# put the result of previous data transformations in an
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# expression, you can use the as-> macro. With it, you can assign a
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# specific name to transformations' output and use it as a
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# placeholder in your chained expressions:
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(as-> [1 2 3] input
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(map inc input) #=> You can use last transform's output at the last position
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(get input 2) #=> and at the second position, in the same expression
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(push [4 5 6] input 8 9 10)) #=> or in the middle !
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# Result: [4 5 6 4 8 9 10]
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# PHP
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#################
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# PHP has a huge and useful standard library, and you're able to use
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# all native functions with the prefix `php/`.
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(php/+ 1 2 3)
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# With :use you can use different namespaces. Similar as `use` in PHP.
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(ns my\module
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(:use \DateTimeImmutable))
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# You can import functions from other phel files with :require
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(ns my\module
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(:require phel\test :refer [deftest is]))
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# Use the class name with a "php/new" to make a new instance
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(php/new \DateTime) # <a date-time object>
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# Use php/-> to call methods of an object
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(def d (php/new \DateTime))
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(php/-> d (getTimestamp)) # <a timestamp>
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# you can do it in one line too
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(php/-> (php/new \DateTime) (getTimestamp))
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# Use php/:: to call static methods
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(php/:: \DateTimeImmutable ATOM) # <a timestamp>
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```
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### Further Reading
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This is far from exhaustive, but hopefully it's enough to get you on your feet.
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Read the full documentation in the website: [https://phel-lang.org/](https://phel-lang.org/documentation/getting-started/)
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