Write You a Haskell Stephen Diehl

Try out some simple examples by declaring some functions at the toplevel of the program. We can
query the types of expressions interactively using the :type command which effectively just runs the
expression halfway through the pipeline and halts after typechecking.
Poly> let id x = x
Poly> let const x y = x
Poly> let twice x = x + x
Poly> :type id
id : forall a. a -> a
Poly> :type const
const : forall a b. a -> b -> a
Poly> :type twice
twice : Int -> Int
Notice several important facts. Our type checker will now flat our reject programs with scoping errors
before interpretation.
Poly> \x -> y
Not in scope: ”y”
Also programs that are also not well-typed are now rejected outright as well.
Poly> 1 + True
Cannot unify types:
Bool
with
Int
e omega combinator will not pass the occurs check.
Poly> \x -> x x
Cannot construct the the infinite type: a = a -> b
e file test.ml provides a variety of tests of the little interpreter. For instance both fact and fib
functions uses the fixpoint to compute Fibonacci numbers or factorials.
let fact = fix (\fact -> \n ->
if (n == 0)
then 1
else (n * (fact (n-1))));
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