Cap 4 Syntax in Functions

Cap 4 Syntax in Functions

1. Pattern matching

   • consists of specifying patterns to which some data should conform and then checking to see if it does and deconstructing the data according to those patterns.
     • When defining functions, you can define separate function bodies for different patterns.
       • the order is important when specifying patterns and it's always best to specify the most specific ones first and then the more general ones later.
         • example
           • length' :: (Num b) => [a] -> b length' [] = 0 length' (_:xs) = 1 + length' xs
             • First we defined the result of a known input — the empty list.
             • Then in the second pattern we take the list apart by splitting it into a head and a tail.
       • we should always include a catch-all pattern so that our program doesn't crash if we get some unexpected input.

2. Guards

   • guards are a way of testing whether some property of a value are true or false
     • A guard is basically a boolean expression.
     • the last guard is otherwise. otherwise is defined simply as otherwise = True and catches everything
     • no use = right after the function name and its parameters, before the first guard.
   • Guards are indicated by pipes that follow a function's name and its parameters.

3. Where

   • Is a condition
     • and we put the keyword where after the guards
       • and then we define several names or functions
         • The names we define in the where section of a function are only visible to that function
       • we usualy aligned at a single column
     • where bindings can also be nested

4. Let

   • are a syntactic construct
     • Let bindings let you bind to variables anywhere and are expressions themselves
       • it can also put let bindings inside list comprehensions
     • The form is let <bindings> in <expression>
   • are very local, so they don't span across guards

5. Case expressions

   • blocks of code for specific values of that variable
   • we can do pattern matching
     • evaluating pieces of code based on its value

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