Question

I want to apply a 3 argument function in different ways based on a boolean value (one of the arguments).

I’d like to be able to apply it in an infix manner so I can chain it (example below). something like the following but that actually works.

f :: Bool -> a -> a -> a

f True  i j = i
f False i j = j

... y `(f True)` z `(f False)` b

Do I need to have the Bool as the second variable? Or something else? I’m kind of lost.

P.S. the reason I wanted to do this was to have optional append function

Answer 1

The infix notation for functions with more than two arguments is possible but not very nice. In your case, given

f :: Bool -> a -> a -> a
f True i j = i
f False i j = j

you can write

(True `f` 1) 3

It’s much easier (as suggested by others) to write:

let ft = f True
let ff = f False

then you can do

1 `ft` 3 `ff` 5

If you want to use f you should write:

(False `f` ((True `f` 1) 3)) 5

You can verify that

1 `ft` 3 `ff` 5 == (False `f` ((True `f` 1) 3)) 5