Haskell Syntax & Key Concepts: Functions, Datatypes, Layout, Comments, Local Defs - Prof. , Study notes of Computer Science

Download Haskell Syntax & Key Concepts: Functions, Datatypes, Layout, Comments, Local Defs - Prof. and more Study notes Computer Science in PDF only on Docsity! COSC 3015: Lecture 5 Lecture given by Prof. Caldwell and scribed by Sunil Kothari September 16, 2008 1 Some Notes on Haskell Syntax Function names, variable names starts with lowercase letter - followed by zero or more digits, underscores ( ) upper or lowercase letters, forward quotes ()́. For example, x1, x′, x1, x′′. Datatype and datatype constructor names start with uppercase letters and are otherwise like functions and variable names. For example, Bool, String, Int , True, False. Keywords case, class, data, default, deriving, do, else, if, import, in , infix, infixl, infixr, instance, let, module, newtype, of , then, type, where. 1.1 Layout rule Each function definition must begin in the same column. a = b + c where b = 1 c = 2 d a = a* 2 a = b + c where {b = 1; c = 2} d a = a* 2 1.2 Comments Line comments start with ”–” and continue to the end of the line. For exam- ple, fx = x + 1−−dumbfunction Nested comments are of the form {− · · · −} 1 1.3 Local Definition The following code has the same effect as the ex. 1 a = let b = 1 c = 2 in b + c Here’s another version of it. a = b + c where b = 1 c = 2 Meaning of let-in. let x = t1 in t2 means t2[x := t1] where, • x is locally defined variable • t1 is local declaration of x • t2 body of the let-declaration and t2[x := t1] - evaluate expression t2 where all free occurrences of x get the value of expression t1. Note that ”:=” is a capture-avoiding substitution let x = 1 in let x = 2 in x ; 2 we can elaborate on it ∀x : Int,∀x : nat.x ≥ 0. Choose arb. y ∈ int and show ∀x : Int,∀x : nat.x ≥ 0[x := y] ; ∀x : nat, x ≥ 0 1.4 if-then-else A code fragment in an imperative language if x > 0 then y := x else y:= (-x) In C and C++ there is a conditional expression 2