Guile scheme cond ERROR: Wrong type to apply

Question

It is not clear to me why this cond gives Wrong type to apply error.

scheme@(guile-user) [12]>(cond ((equal? "i" "i") => (display "yay")))

yay

ERROR: In procedure #:

ERROR: Wrong type to apply: #

scheme@(guile-user) [12]>(cond ((string= "i" "i") => (display "yay")))

yay

ERROR: In procedure #:

ERROR: Wrong type to apply: #

Óscar López Óscar López · Accepted Answer · 2016-06-21T16:24:27

The usual syntax for cond is as follows:

(cond ((equal? "i" "i")
       (display "yay")))
; prints yay

We use => when we want to pass the result of the condition as a parameter to a function that gets executed, for instance:

(cond ((equal? "i" "i") 
       => display))
; prints #t

In the above code the condition evaluates to #t, and #t is passed as a parameter to display, which prints it.