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Unformatted text preview: Click to edit Master subtitle style Chapter Nine Modern Programming Languages, 2nd ed. A Third Look At ML Chapter Nine Modern Programming Languages, 2nd ed. 11 Chapter Nine Modern Programming Languages, 2nd ed. Outline ■ More pattern matching ■ Function values and anonymous functions ■ Higherorder functions and currying ■ Predefined higherorder functions Chapter Nine Modern Programming Languages, 2nd ed. 22 Chapter Nine Modern Programming Languages, 2nd ed. More PatternMatching ■ Last time we saw patternmatching in function definitions: – fun f 0 = "zero"  f _ = "nonzero"; ■ Patternmatching occurs in several other kinds of ML expressions: – case n of 0 => "zero"  _ => "nonzero"; Chapter Nine Modern Programming Languages, 2nd ed. 33 Chapter Nine Modern Programming Languages, 2nd ed. Match Syntax ■ A rule is a piece of ML syntax that looks like this: ■ A match consists of one or more rules separated by a vertical bar, like this: ■ Each rule in a match must have the same type of expression on the righthand side ■ A match is not an expression by itself, but forms a part of several kinds of ML expressions Chapter Nine Modern Programming Languages, 2nd ed. 44 < rule > ::= < pattern > => < expression > < match > ::= < rule >  < rule > '  ' < match > Chapter Nine Modern Programming Languages, 2nd ed. Case Expressions ■ The syntax is ■ This is a very powerful case construct—unlike Chapter Nine Modern Programming Languages, 2nd ed. 55 case 1+1 of = 3 => "three"  = 2 => "two"  = _ => "hmm"; val it = "two" : string < caseexpr > ::= case < expression > of < match > Chapter Nine Modern Programming Languages, 2nd ed. Example Chapter Nine Modern Programming Languages, 2nd ed. 66 case x of _::_::c::_ => c  _::b::_ => b  a::_ => a  nil => 0 The value of this expression is the third element of the list x , if it has at least three, or the second element if x has only two, or the first element if x has only one, or 0 if x is empty. Chapter Nine Modern Programming Languages, 2nd ed. Generalizes if ■ The two expressions above are equivalent ■ So if then else is really just a special case of case Chapter Nine Modern Programming Languages, 2nd ed. 77 if exp 1 then exp 2 else exp 3 case exp 1 of true => exp 2  false => exp 3 Chapter Nine Modern Programming Languages, 2nd ed. Outline ■ More pattern matching ■ Function values and anonymous functions ■ Higherorder functions and currying ■ Predefined higherorder functions Chapter Nine Modern Programming Languages, 2nd ed. 88 Chapter Nine Modern Programming Languages, 2nd ed. Predefined Functions ■ When an ML language system starts, there are many predefined variables ■ Some are bound to functions: Chapter Nine Modern Programming Languages, 2nd ed....
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 Spring '11
 Hamlen
 Functional Programming, Currying, val f, Modern Programming

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