May 09, 2021 CoffeeScript
Others need to run some of your code in a controlled manner. In contrast, the language you choose cannot express the problem domain in a concise way.
Use interpreter patterns to create a domain-specific language that you translate into specific code.
Let's make a hypothesis, for example, that the user wants to perform mathematical operations in your application. Y ou can have them running code in the right way to calculate instructions (eval), but this will let them run any code. Instead, you can provide a small "stack calculator" language for individual analysis to run only mathematical operations while reporting more useful error messages.
class StackCalculator
parseString: (string) ->
@stack = [ ]
for token in string.split /\s+/
@parseToken token
if @stack.length > 1
throw "Not enough operators: numbers left over"
else
@stack[0]
parseToken: (token, lastNumber) ->
if isNaN parseFloat(token) # Assume that anything other than a number is an operator
@parseOperator token
else
@stack.push parseFloat(token)
parseOperator: (operator) ->
if @stack.length < 2
throw "Can't operate on a stack without at least 2 items"
right = @stack.pop()
left = @stack.pop()
result = switch operator
when "+" then left + right
when "-" then left - right
when "*" then left * right
when "/"
if right is 0
throw "Can't divide by 0"
else
left / right
else
throw "Unrecognized operator: #{operator}"
@stack.push result
calc = new StackCalculator
calc.parseString "5 5 +" # => { result: 10 }
calc.parseString "4.0 5.5 +" # => { result: 9.5 }
calc.parseString "5 5 + 5 5 + *" # => { result: 100 }
try
calc.parseString "5 0 /"
catch error
error # => "Can't divide by 0"
try
calc.parseString "5 -"
catch error
error # => "Can't operate on a stack without at least 2 items"
try
calc.parseString "5 5 5 -"
catch error
error # => "Not enough operators: numbers left over"
try
calc.parseString "5 5 5 foo"
catch error
error # => "Unrecognized operator: foo"
As an alternative to writing our own interpreters, you can combine existing CoffeeScript interpreters with the normal way more natural (and understandable) algorithms express themselves.
class Sandwich
constructor: (@customer, @bread='white', @toppings=[], @toasted=false)->
white = (sw) ->
sw.bread = 'white'
sw
wheat = (sw) ->
sw.bread = 'wheat'
sw
turkey = (sw) ->
sw.toppings.push 'turkey'
sw
ham = (sw) ->
sw.toppings.push 'ham'
sw
swiss = (sw) ->
sw.toppings.push 'swiss'
sw
mayo = (sw) ->
sw.toppings.push 'mayo'
sw
toasted = (sw) ->
sw.toasted = true
sw
sandwich = (customer) ->
new Sandwich customer
to = (customer) ->
customer
send = (sw) ->
toastedState = sw.toasted and 'a toasted' or 'an untoasted'
toppingState = ''
if sw.toppings.length > 0
if sw.toppings.length > 1
toppingState = " with #{sw.toppings[0..sw.toppings.length-2].join ', '} and #{sw.toppings[sw.toppings.length-1]}"
else
toppingState = " with #{sw.toppings[0]}"
"#{sw.customer} requested #{toastedState}, #{sw.bread} bread sandwich#{toppingState}"
send sandwich to 'Charlie' # => "Charlie requested an untoasted, white bread sandwich"
send turkey sandwich to 'Judy' # => "Judy requested an untoasted, white bread sandwich with turkey"
send toasted ham turkey sandwich to 'Rachel' # => "Rachel requested a toasted, white bread sandwich with turkey and ham"
send toasted turkey ham swiss sandwich to 'Matt' # => "Matt requested a toasted, white bread sandwich with swiss, ham and turkey"
This instance allows the functional layer implementation to return the modified object so that the external function can modify it in turn. T he example provides the structure with natural syntax by borrowing the usage of verbs and prepositions, and when used correctly, it ends like a natural statement. In this way, using CoffeeScript language skills and your existing language skills can help you with your questions about capturing code.