Add realize function for converting fractions to dividable types

docs/intro-to-coalton.md

@@ -546,6 +546,8 @@ All of these cases are sufficiently common that we provide a few shorthands:
 
 - `floor/`, `ceiling/`, and `round/` for integer-to-integer division, and
 
+- `fromfrac` for converting a `Fraction` into a type with a `Dividable Integer :a` instance (this conversion may result in precision loss)
+
 ## Lists
 
 Coalton uses Lisp lists under the hood. Lists can be constructed with `make-list`.

library/math/real.lisp

@@ -31,7 +31,8 @@
    #:inexact/
    #:floor/
    #:ceiling/
-   #:round/))
+   #:round/
+   #:fromfrac))
 
 (in-package #:coalton-library/math/real)
 
@@ -264,7 +265,14 @@ Note: This does *not* divide double-float arguments."
   (declare round/ (Integer -> Integer -> Integer))
   (define (round/ a b)
     "Divide two integers and round the quotient."
-    (round (exact/ a b))))
+    (round (exact/ a b)))
+
+  (declare fromfrac (Dividable Integer :a => Fraction -> :a))
+  (define (fromfrac q)
+    "Converts a fraction to a type `:a` with an instance of `Dividable Integer :a`, similar to `Num`'s `fromint`.
+
+This may result in loss of fidelity."
+    (general/ (numerator q) (denominator q))))
 
 #+sb-package-locks
 (sb-ext:lock-package "COALTON-LIBRARY/MATH/REAL")