Update spec.md

1 files changed, 7 insertions, 1 deletions

diff --git a/spec.md b/spec.md
index 505524d..1b25e6b 100644
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+++ b/spec.md
@@ -5,7 +5,7 @@
 - Boolean functions marked with ? (convention)
 - Loops implemented with recursion
 
-## Value types
+## Values and types
 
     Integer     3
     Float       -3.14
@@ -17,6 +17,12 @@
     Hash map    { a 1 b 2 }
     Function    (\[x y] (+ x y))
 
+Functions generally only accept monomorphic values, but some builtins are polymorphic, either expecting any arbitrary type (as is the case in e.g. `match` and `fmt`), or only a range of types (type classes). User defined functions can be made "polymorphic" by matching on the return value of `(type arg)`, where `arg` is the argument of the user defined function.
+
+There are 2 language defined "type classes": `Num` and `Seq`. Integers and floats are instances of `Num`, while Strings and Vectors are instances of `Seq`. Type classes are not a thing per se in the interpreter, but they are a useful concept to "formalize" some builtin functions and their inputs. `Num` values can be e.g. added together with `+`. `Seq` values can be used to call functions like `head` and `tail` and `len`.
+
+Collections (vectors, hash maps) are heterogenous, which means that a single collection can contains values of multiple types.
+
 ## Syntax
 
 ### Function calls