The coersion to InputForm produces a lispy representation for most types. E.g.

(1) -> )set output tex off
 

)set output algebra on

p:POLY INT:=x^2+1

         2
   (1)  x  + 1

pSex:=p::InputForm

   (2)  (+ (^ x 2) 1)

p1:=interpret(pSex)

         2
   (3)  x  + 1

But this fails for functions

f:INT->INT

f(x) == x^2+1

-- force compile
f(2)

Compiling function f with type Integer -> Integer 

   (6)  5

fSex:=f::InputForm

   (7)
   (coerceOrCroak
    (CONS '(Mapping (Integer) (Integer)) (wrap (SPADMAP (#1 + (^ #1 2) 1))))
    '(InputForm)  'noMapName)

So far so good, but

f1:=interpret(fSex)

   Cannot convert the value from type (Integer -> Integer) to InputForm
      .