FParsec Hands On - F#unctional Londoners 2014

FPARSEC HANDS ON
Phillip Trelford
F#unctional Londoners,
2014

INTERNAL DSLS - EXAMPLES
Fake
#r "tools/FAKE/tools/FakeLib.dll"
open Fake
Target "Test" (fun _ ->
trace "Testing stuff..."
)
Target "Deploy" (fun _ ->
trace "Heavy deploy action"
)
"Test" // define the dependencies
==> "Deploy"
Run "Deploy"
Canopy
"starting a game of 2048" &&& fun _ ->
start firefox
url "http://gabrielecirulli.github.io/2048/"
let score = element ".score-container"
printfn "Score %s" score.Text
while not(lost()) && not (won()) do
press up
press right
press left
press up

EXTERNAL DSLS IN ACTION
1. Domain-specific syntax
2. Proprietary encodings
3. Custom execution
environments

BUILDING EXTERNAL DSLS
Abstract Syntax Tree (AST)
Parse
Execute

PARSING OPTIONS
Hand rolled
•Regex
•Active
Patterns
FParsec
•Combinators
•Monadic
FsLex, FsYacc
•Lexer
•Parser

FPARSEC
FParsec Hands On
F#unctional Londoners
2014

THINGS YOU SHOULD NEVER DO
Custom Operators
Funny picture

FUNSCRIPT’S TYPESCRIPT PARSER
FParsec
let interfaceDeclaration =
str_ws "interface" >>. identifier .>>. typeParams
.>>. interfaceExtendsClause .>>. objectType
|>> fun (((id, tps), ex), t) ->
InterfaceDeclaration(id, tps, ex, t)
TypeScript Definition
interface JQueryKeyEventObject
extends JQueryInputEventObject {
char: any;
charCode: number;
key: any;
keyCode: number;
}

FPARSEC TUTORIAL
open FParsec
let test p str =
match run p str with
| Success(result, _, _) -> printfn "Success: %A" result
| Failure(errorMsg, _, _) -> printfn "Failure: %s" errorMsg
test pfloat "1.25"
test pfloat "1.25E 2"
let str s = pstring s
let floatBetweenBrackets = str "[" >>. pfloat .>> str "]"
test floatBetweenBrackets "[1.0]"
test floatBetweenBrackets "[]"
test floatBetweenBrackets "[1.0]"

PARSING OPERATORS
// we set up an operator precedence parser for parsing the arithmetic expressions
let opp = new OperatorPrecedenceParser<float,unit,unit>()
let expr = opp.ExpressionParser
opp.TermParser <- number <|> between (str_ws "(") (str_ws ")") expr
// operator definitions follow the schema
// operator type, string, trailing whitespace parser, precedence, associativity, function to apply
opp.AddOperator(InfixOperator("+", ws, 1, Associativity.Left, (+)))
opp.AddOperator(InfixOperator("-", ws, 1, Associativity.Left, (-)))
opp.AddOperator(InfixOperator("*", ws, 2, Associativity.Left, (*)))
opp.AddOperator(InfixOperator("/", ws, 2, Associativity.Left, (/)))
opp.AddOperator(InfixOperator("^", ws, 3, Associativity.Right, fun x y -> System.Math.Pow(x, y)))
opp.AddOperator(PrefixOperator("-", ws, 4, true, fun x -> -x))

TURTLES ALL THE WAY
DOWN
FParsec Hands On
2014

TURTLE LANGUAGE
repeat 10
[right 36 repeat 5
[forward 54 right 72]]

TURTLE AST
module AST
type arg = int
type command =
| Forward of arg
| Turn of arg
| Repeat of arg * command list

TURTLE PARSER: FORWARD AND
TURN
open AST
open FParsec
let pforward =
pstring "forward" >>. spaces1 >>. pfloat
|>> fun x -> Forward(int x)
let pleft =
pstring "left" >>. spaces1 >>. pfloat
|>> fun x -> Turn(int -x)
let pright =
pstring "right" >>. spaces1 >>. pfloat
|>> fun x -> Turn(int x)

TURTLE PARSER: REPEAT
COMMAND
let pcommand = pforward <|> pleft <|> pright
let block = between (pstring "[") (pstring "]") (many1 (pcommand .>> spaces))
let prepeat =
pstring "repeat" >>. spaces1 >>. pfloat .>> spaces .>>. block
|>> fun (n,commands) -> Repeat(int n, commands)

TURTLE PARSER: FORWARD
REPEAT
let prepeat, prepeatimpl = createParserForwardedToRef ()
let pcommand = pforward <|> pleft <|> pright <|> prepeat
let block = between (pstring "[") (pstring "]") (many1 (pcommand .>> spaces))
prepeatimpl :=
pstring "repeat" >>. spaces1 >>. pfloat .>> spaces .>>. block
|>> fun (n,commands) -> Repeat(int n, commands)

TURTLE INTERPRETER: PATTERN
MATCHING
let rec perform turtle = function
| Forward n ->
let r = float turtle.A * Math.PI / 180.0
let dx, dy = float n * cos r, float n * sin r
let x, y = turtle.X, turtle.Y
let x',y' = x + dx, y + dy
drawLine (x,y) (x',y')
{ turtle with X = x'; Y = y' }
| Turn n -> { turtle with A = turtle.A + n }
| Repeat (n, commands) ->
let rec repeat turtle = function
| 0 -> turtle
| n -> repeat (performAll turtle commands) (n-1)
repeat turtle n
and performAll = List.fold perform

TURTLE: PROCEDURES
to square
repeat 4 [forward 50 right 90]
end
to flower
repeat 36 [right 10 square]
end
to garden
repeat 25 [set-random-position flower]
end
garden

SMALL BASIC COMPILER
FParsec Hands On
2014

SMALL BASIC IDE
FParsec Hands On
2014

SMALL BASIC
SAMPLE
Sub Init
gw = 598
gh = 428
GraphicsWindow.BackgroundColor =
"DodgerBlue"
GraphicsWindow.Width = gw
GraphicsWindow.Height = gh
color = "1=Orange;2=Cyan;3=Lime;"
size = "1=20;2=16;3=12;"
passed = 0
cd = "False" ' collision detected
EndSub

SMALL BASIC PARSER
/// > For i = 1 To 100 Step 2
let pfor =
let pfrom = str_ws1 "For" >>. pset
let pto = str_ws1 "To" >>. pexpr
let pstep = str_ws1 "Step" >>. pexpr
let toStep = function None -> Literal(Int(1)) | Some s -> s
pipe3 pfrom pto (opt pstep) (fun f t s -> For(f, t, toStep s))
let pendfor = str_ws "EndFor" |>> (fun _ -> EndFor)

SMALL BASIC COMPILER: GOTO
| Goto(name) ->
let label = obtainLabel il name
il.Emit(OpCodes.Br, label)
| Label(name) ->
let label = obtainLabel il name
il.MarkLabel(label)

RESOURCES
FParsec Hands On
2014

F# KOANS
[<Koan>]
let SquareEvenNumbersWithPipelineOperator() =
(* In F#, you can use the pipeline operator to get the benefit of
the parens style with the readability of the statement style. *)
let result =
[0..5]
|> List.filter isEven
|> List.map square
AssertEquality result __

QUESTIONS
Twitter: @ptrelford
Blog: http://trelford.com/blog
Turtle: http://fssnip.net/nN

FParsec Hands On - F#unctional Londoners 2014

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