I am trying to calculate n fibonacci numbers. This works, but i want to print every number calculated on the way to n, which seems to manipulate the return type of the function.
I'm a haskell beginner so i dont know what to search the web for.
fib n =
case n of
0 -> 0
1 -> 1
_ ->
fib (n - 1) + fib (n - 2)
main = do
print (fib 30)
I tried the following, after which the compiler told me im not doing it right:
import Debug.Trace
fib n =
case n of
0 -> 0
1 -> 1
_ -> do
let r = fib (n - 1) + fib (n - 2)
trace (show r)
r
main = do
print (fib 30)
The error in question:
[1 of 1] Compiling Main ( fib.hs, fib.o )
fib.hs:13:3: error:
• No instance for (Show (a0 -> b0)) arising from a use of ‘print’
(maybe you haven't applied a function to enough arguments?)
• In a stmt of a 'do' block: print (fib 30)
In the expression: do print (fib 30)
In an equation for ‘main’: main = do print (fib 30)
|
13 | print (fib 30)
| ^^^^^
fib.hs:13:10: error:
• No instance for (Num (a0 -> b0)) arising from a use of ‘fib’
(maybe you haven't applied a function to enough arguments?)
• In the first argument of ‘print’, namely ‘(fib 30)’
In a stmt of a 'do' block: print (fib 30)
In the expression: do print (fib 30)
|
13 | print (fib 30)
| ^^^
Don't use do, do is used in monadic contexts. You can work with trace although this is merely for debugging purposes with:
import Debug.Trace(traceShowId)
fib n =
case n of
0 -> 0
1 -> 1
_ -> let r = fib (n - 1) + fib (n - 2) in traceShowId r
main = print (fib 30)probably it is more elegant to write the patterns in the "head" of the clauses:
import Debug.Trace(traceShowId)
fib 0 = 0
fib 1 = 1
fib n = let r = fib (n - 1) + fib (n - 2) in traceShowId r
main = print (fib 30)