Racket applies input types to function

Question

I would like to apply a data type to input a function in a racket. For example, in this function I want to expect integers and give an error if someone enters a string.

Is there a standard way to enforce this on Racket?

(sum-coins pennies nickels dimes quarters) pennies : integer? nickels : integer? dimes : integer? quarters : integer? 

Answer 1

Yes, there are two standard ways.

Contracts

Contracts check conditions and cause runtime errors:

 #lang racket (define/contract (sum-coins pennies nickels dimes quarters) (-> integer? integer? integer? integer? integer?) (+ (* 1 pennies) (* 5 nickels) (* 10 dimes) (* 25 quarters))) (sum-coins 1 2 3 4) ;; => 141 (sum-coins "1" 2 3 4) ; sum-coins: contract violation ; expected: integer? ; given: "1" ; in: the 1st argument of ; (-> ; integer? ; integer? ; integer? ; integer? ; integer?) ; contract from: (function sum-coins) ; blaming: /tmp/so.rkt ; (assuming the contract is correct) ; at: /tmp/so.rkt:3.18 ; Context (errortrace): ; /tmp/so.rkt:11:0: (sum-coins "1" 2 3 4) ; /tmp/so.rkt:11:0: (sum-coins "1" 2 3 4) 

Typical rocket

Or you can use Typed Racket , which checks types at compile time:

 #lang typed/racket (define (sum-coins [pennies : Integer] [nickels : Integer] [dimes : Integer] [quarters : Integer]) (+ (* 1 pennies) (* 5 nickels) (* 10 dimes) (* 25 quarters))) (sum-coins 1 2 3 4) ;; => 141 (sum-coins "1" 2 3 4) ; /tmp/so2.rkt:14:11: Type Checker: type mismatch ; expected: Integer ; given: String ; in: "1"