testing_interpreter.py [-v|(-p|-n)] -f file_to_execute

   -v   call by value (by default is the strategy `call by name`)
   -p   print each reduction (default)
   -n   not print reductions (override `-p`)

$ # see: https://en.wikipedia.org/wiki/Lambda_calculus#Recursion_and_fixed_points
$ testing_interpreter.py -f examples/factorial.lambda
Y fac 2

= Y fac 2
= (λf.(λx.x x) (λx.f (x x))) fac 2
= ((λx.x x) (λx.fac (x x))) 2 // β-reduc
= (λx.x x) (λx.fac (x x)) 2 // left assoc λ aplications: (f x) y = f x y
= ((λx.fac (x x)) (λx.fac (x x))) 2 // β-reduc
= (λx.fac (x x)) (λx.fac (x x)) 2 // left assoc λ aplications: (f x) y = f x y
= (fac ((λx.fac (x x)) (λx.fac (x x)))) 2 // β-reduc
= fac ((λx.fac (x x)) (λx.fac (x x))) 2 // left assoc λ aplications: (f x) y = f x y
= (λf.λx.if (x=0) 1 (x*f (x-1))) ((λx.fac (x x)) (λx.fac (x x))) 2
= (λx.if (x=0) 1 (x*((λx.fac (x x)) (λx.fac (x x))) (x-1))) 2 // β-reduc
= if (2=0) 1 (2*((λx.fac (x x)) (λx.fac (x x))) (2-1)) // β-reduc
 if|= 2=0
 if|= false
= 2*((λx.fac (x x)) (λx.fac (x x))) (2-1)
  r-> ((λx.fac (x x)) (λx.fac (x x))) (2-1)
  |= ((λx.fac (x x)) (λx.fac (x x))) (2-1)
  |= (λx.fac (x x)) (λx.fac (x x)) (2-1) // left assoc λ aplications: (f x) y = f x y
  |= (fac ((λx.fac (x x)) (λx.fac (x x)))) (2-1) // β-reduc
  |= fac ((λx.fac (x x)) (λx.fac (x x))) (2-1) // left assoc λ aplications: (f x) y = f x y
  |= (λf.λx.if (x=0) 1 (x*f (x-1))) ((λx.fac (x x)) (λx.fac (x x))) (2-1)
  |= (λx.if (x=0) 1 (x*((λx.fac (x x)) (λx.fac (x x))) (x-1))) (2-1) // β-reduc
  |= if (2-1=0) 1 ((2-1)*((λx.fac (x x)) (λx.fac (x x))) (2-1-1)) // β-reduc
  | if|= 2-1=0
  | if|  l-> 2-1
  | if|  |= 2-1
  | if|  |= 1
  | if|= false
  |= (2-1)*((λx.fac (x x)) (λx.fac (x x))) (2-1-1)
  |  l-> 2-1
  |  |= 2-1
  |  |= 1
  |  r-> ((λx.fac (x x)) (λx.fac (x x))) (2-1-1)
  |  |= ((λx.fac (x x)) (λx.fac (x x))) (2-1-1)
  |  |= (λx.fac (x x)) (λx.fac (x x)) (2-1-1) // left assoc λ aplications: (f x) y = f x y
  |  |= (fac ((λx.fac (x x)) (λx.fac (x x)))) (2-1-1) // β-reduc
  |  |= fac ((λx.fac (x x)) (λx.fac (x x))) (2-1-1) // left assoc λ aplications: (f x) y = f x y
  |  |= (λf.λx.if (x=0) 1 (x*f (x-1))) ((λx.fac (x x)) (λx.fac (x x))) (2-1-1)
  |  |= (λx.if (x=0) 1 (x*((λx.fac (x x)) (λx.fac (x x))) (x-1))) (2-1-1) // β-reduc
  |  |= if (2-1-1=0) 1 ((2-1-1)*((λx.fac (x x)) (λx.fac (x x))) (2-1-1-1)) // β-reduc
  |  | if|= 2-1-1=0
  |  | if|  l-> 2-1-1
  |  | if|  |= 2-1-1
  |  | if|  |  l-> 2-1
  |  | if|  |  |= 2-1
  |  | if|  |  |= 1
  |  | if|  |= 0
  |  | if|= true
  |  |= 1
  |= 1
= 2

= 2

$ testing_interpreter.py -nf examples/factorial.lambda
2

$ # see: https://en.wikipedia.org/wiki/99_Bottles_of_Beer
$ testing_interpreter.py -vnf examples/beer.lambda

99 bottles of beer on the wall, 99 bottles of beer.
Take one down and pass it around, 98 bottles of beer on the wall.

98 bottles of beer on the wall, 98 bottles of beer.
Take one down and pass it around, 97 bottles of beer on the wall.

...

1 bottle of beer on the wall, 1 bottle of beer.
Take one down and pass it around, no more bottles of beer on the wall.

No more bottles of beer on the wall, no more bottles of beer.
Go to the store and buy some more, 99 bottles of beer on the wall.