Beginner's question

[Cyrille DEUSS]

#lang racket
(require racket/format)

(define pp_number
  (lambda (n)
     (~a n #:width 6 #:align 'right  #:left-pad-string "0")))

(define fp
  (lambda (s n)
    (string-append (pp_number n) " + " s)))

(define f
  (lambda (s n)
    (format "~a + ~a" n s)))

(foldl f "" '(1 2 3))
; works great !

(foldl fp "" '(1 2 3))
; and why not this one ?

Thanks in advance.

Cyrille

[Jens Axel Søgaard]

Try reversing the order for arguments for both fp and f.

/Jens Axel

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--
Jens Axel Søgaard

[David Storrs]

Jens answered your question, but I'll note two convenient shorthands:

> #lang racket

> (require racket/format)

racket/format is provided by #lang racket, so you can skip that.

This:

(define fp
  (lambda (s n)
    (string-append (pp_number n) " + " s)))

is the same as this:

(define (fp s n)

    (string-append (pp_number n) " + " s)))

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[Cyrille DEUSS]

As usual I didn't read the manual which is a little bit cryptic for a non native speaker.

The signature of fodl is ('a -> 'b -> 'b) -> 'b -> 'a list -> 'b

I was used to the signature of List.fold_left of OCaml where the signature is : ('a -> 'b -> 'a) -> 'a -> 'b list -> 'a

The arguments are reversed...

Thx