Merge pull request exercism#294 from exercism/reference/exercise-conc…

…epts/bob.md

Extract Concepts from v2 exercise: bob

reference/exercise-concepts/bob.md

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+# Concepts of bob
+
+[Example implementation](https://github.com/exercism/fsharp/blob/master/exercises/bob/Example.fs)
+
+## General
+
+- functions: used as the main entry point for the exercise
+- function arguments: input strand is passed as an argument
+- methods: calling the `Char.IsLetter`, `String.IsNullOrEmpty` method and several other `string` methods
+- return values: returning a value from a function
+- implicit returns: the last expression is automatically returned from a function
+- type annotations: annotating the string parameter
+- type inference: automatically infer the type of the functions and values
+- scoping: using whitespace to define scope
+- modules: the functions are defined in a module
+- imports: import types through `open` statements
+- namespaces: knowing where to find the `Char` class
+- assignment: assigning values
+- higher order functions: passing `Char.IsLetter` as an argument to `Seq.exists`
+- collection filtering: using `Seq.exists` to see if the string contains a letter
+- equality operators: `=` used to compare strings
+- boolean operators: `&&` used to combine boolean expressions
+- strings: a `string` passed as the single input parameter
+- booleans: a `bool` is used as the return value of the various string type tests
+
+## Approach: if-else
+
+- conditionals using if-elif-else: conditionally execute logic using an `if/elif/else` statement
+
+## Approach: inner functions
+
+- nested functions: defining helper functions inside tested function
+
+## Approach: active patterns
+
+- active patterns: defining active patterns for the various responses
+- pattern matching: matching on the active patterns
+
+## Approach: pattern matching
+
+- pattern matching: matching on the input
+- guard clauses: adding guard clauses to the pattern matching
+- discards: ignore the pattern matches for the guard clauses to work
+
+## Approach: regular expressions
+
+- conditionals using if-elif-else: conditionally execute logic using an `if/elif/else` statement
+- regular expressions: checking the phrase types using regular expressions