Ideas for a toy programming language project

gistfile1.txt

fib(n) =
    if n <= 1
    then n  // TODO: is "then" keyword required? not strictly but improves readability?
    else
        left = fib(n-2)
        right = fib(n-1)
        left + right

fib_iter(n) =
    iter(a, b, n) =
        if n == 0 then a
        elif n == 1 then b
        else iter(b, a+b, n-1)
    iter(0, 1, n)

example_declarations(x) =
    // NOTE: declarations are like letrec expressions in Scheme/ML/Haskell
    // declarations are *not* like assignments in C or Python
    f(w) = w * (x + y) * z
    y = z + x
    z = 42
    f(x)

mutual_recursion(x) =
    even(x) = if x == 0 then true else odd(x-1)
    odd(x) = if x == 0 then false else even(x-1)
    even(x) != odd(x)

mutual_recursion2(x) =
    // short circuiting (?)
    even(x) = x == 0 || odd(x-1)
    odd(x) = x == 1 || even(x-1)
    even(x) != odd(x)

semicolons_are_for_seqencing() =
    print("hello "); print("world\n") // NOTE: no semicolon allowed in the end!

semicolons_are_for_seqencing2() =
    // prints: why y\n 43 55
    x = 1
    y = print("why y\n"); 42
    print(x + y);
    x = 13      // NOTE: this is *not* an re-assignment
    print(x + y)

semicolons_are_for_seqencing3() =
    // prints: "x\ny\n" OR "y\nx\n" (no semicolon, no ordering)
    x = print("x\n"); 42
    y = print("y\n"); 13
    x + y

semicolons_are_for_seqencing4() =
    // prints: "x\ny\n" IN THAT ORDER
    x = print("x\n"); 42
    pass; // add a "dummy" sequencing point
    y = print("y\n"); 13
    x + y

semicolons_are_for_seqencing5() =
    // this is a contrived example, don't write code like this!
    // prints: 42 13
    x = 42
    y = print(x); x = 13 print(x); x+1
    x+y     // returns: (42+14) = 56

higher_order(cmp, a b) =
    // type: forall a. ((a, a) -> bool, a, a) -> a
    if cmp(a, b) then a else b

// here's some legal but awkward examples
fucked_up1() =
    // TODO: should there be parenthesis required around if-then-else
    f(if a then b else c, d)

fucked_up2() =
    // let declarations inside a function call are awkward too (add parens?)
    f(x = 42 y = 13 x+y, z)

fucked_up3() =
    // confused yet?
    f(print("hello"); print("world"); x+y, z)

fucked_up4() =
    // can even parenthesis save this one?
    f(if foo(); bar then quux else x = 42 y = 13 print(x); print(y); x*y, z)