Parsing with Blocks CocoaHeads Hamburg Chris Eidhof 

Warning All of this is a bit crazy 

Building languages is magical 

stylesheet { multiplier = if(slide.onlyHeader, 2, 1), baseFontSize = 45 * multiplier, splitBaseFontSize = 25, backgroundColor = colorScheme.backgroundColor, bodyFont = { family = "Apercu", color = colorScheme.textColor, size = baseFontSize } } 

DSLs → SQL → AutoLayout → NSPredicate → Regular Expressions → ActiveRecord 

DSLs External vs. Internal 

DSLs: External → SQL → CSS → NSPredicate format syntax 

DSLs: Internal [view1 mas_makeConstraints:^(MASConstraintMaker *make) { make.top.equalTo(superview.mas_top).with.offset(padding.top); make.left.equalTo(superview.mas_left).with.offset(padding.left); make.bottom.equalTo(superview.mas_bottom).with.offset(-padding.bottom); make.right.equalTo(superview.mas_right).with.offset(-padding.right); }] Source: Masonry 

V:[topField]-10-[bottomField] [flexibleButton(>=70,<=100)] |-[find]-[findNext]-[findField(>=20)]-| 

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Building an external DSL 

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Parsing 1. String → Stream of tokens 2. Stream of tokens → Syntax Tree 3. Interpret Syntax Tree 

Existing approaches → Hand-rolled parsers → Parser generators → ParseKit / CoreParse 

An alternative approach → Based on functional programming → Backtracking → Immutable objects 

The language 

1 + 2 * 3 Expr ← Sum Sum ← Product '+' Product Product ← Atom '*' Atom Atom ← Number 

1 + 2 * 3 Expr ← Sum Sum ← Product '+' Product | Product Product ← Atom '*' Atom | Atom Atom ← Number 

(1 + 2) * 3 

(1 + 2) * 3 Expr ← Sum Sum ← Product '+' Product | Product Product ← Atom '*' Atom | Atom Atom ← Number | '(' Expr ')' 

Block Syntax returnType (^blockName)(parameterTypes) = ^returnType(parameters) {...}; Source: http://goshdarnblocksyntax.com 

Live Coding 

A stylesheet language 

stylesheet { multiplier = if(slide.onlyHeader, 2, 1), baseFontSize = 45 * multiplier, splitBaseFontSize = 25, backgroundColor = colorScheme.backgroundColor, bodyFont = { family = "Apercu", color = colorScheme.textColor, size = baseFontSize } } 

Rule stylesheet = ^(State *p) { __block NSArray *functions = nil; return p.identifier().token(@"{"). manySepBy(method, comma).bind(to(functions)). token(@"}").yield(^id { return [[StylesheetObject alloc] initWithFunctions:functions]; }); }; 

Rule method = ^(State * p) { __block NSString* name = nil; __block id body= nil; __block NSArray *parameters = nil; return p.identifier().bind(to(name)). optional(parameterList).bind(to(parameters)). token(@"="). rule(self.expression).bind(to(body)). yield(^id { return [[StylesheetFunction alloc] initWithName:name body:body parameters:parameters]; }); }; 

Rule mulExpression = infix(@"*", infix(@"/", functionCallExpression)); Rule addExpression = infix(@"+", infix(@"-", mulExpression)); 

API 

typedef State *(^Rule)(State *p); 

@interface State : NSObject @property (nonatomic, strong) id result; @property (nonatomic) BOOL failed; @property (nonatomic) NSString *errorMessage; @property (nonatomic) State *(^token)(NSString *); @property (nonatomic) State *(^identifier)(); @property (nonatomic) State *(^yield)(id (^)()); @property (nonatomic) State *(^bind)(void(^)(id result)); @property (nonatomic) State *(^oneOf)(NSArray *); @property (nonatomic) State *(^optional)(Rule); @property (nonatomic) State *(^eof)(); @property (nonatomic) State *(^rule)(Rule); @end 

State Internals @interface State () @property (nonatomic) NSArray *tokens; @property (nonatomic) NSUInteger tokenIndex; @end 

self.token = ^(NSString* token) { if (self.failed) return self; NSString* peek = self.peek; if ([token isEqual:peek]) { return [self next:peek]; } else { NSString* msg = [NSString stringWithFormat:@"Expected '%@', saw '%@'", token, peek]; return [self fail:msg]; } }; 

- (State *)next:(id)result { if (self.failed) return self; State *next = [self copy]; next.failed = NO; next.result = result; next.tokenIndex++; return next; } 

We created two DSLs → One for parsing (embedded) → One external https://github.com/chriseidhof/parsingwithblocks 

Thanks → @chriseidhof → http://www.objc.io → http://www.uikonf.com → http://www.decksetapp.com