templates stuff

3ef5c4cf9f23875b9e2111946449e410b9ab3317 · ovidiucs · Feb 18, 2019 21:46
+53 · 1 file changed
templates_stuff.nim +53 −0
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+# Copyright 2019.
+## About templates:
+## * A template operates on Nim's abstract syntax trees.
+## * A template generates procedures.
+## * A template is injected into the main scope as is.
+## * A tempalte is processed in the semantic pass of the compiler.
+## * A template in nim is a simple declarative macro
+## * It provides a substitution mechanism that allows you to substitute
+##   arguments passed to them in the body of the template.
+## * Used to reduce code duplication.
+## * Uset to inject variables into the calling scope.
+## * Are invoked just as a procedure.
+## * Templates are declarative.
+## * A template is kind of like a type-safe ``#define``.
+## * A template is a simple quasi-quote.
+## * Evaluation is done at compile-time.
+## * A template is not a proc. return doesn't work as you expect it to.
+## * A template is a way to use one function in multiple scenarios.
+
+## Example:
+##
+## .. code-block:: Nim
+##  template foo(body: untyped) =
+##   body
+##
+##  foo():
+##   echo "Bla"
+##   echo "Blo"
+
+## Outputs:
+##
+## .. code-block:: Nim
+##   Bla
+##   Blo
+template foo(body: untyped) =
+  body
+
+foo():
+  echo "Bla"
+  echo "Blo"
+## .. code-block:: Nim
+##
+##   template `!=` (a,b: untyped): untyped =
+##   # this definition exists int the system module
+##     not (a == b)
+##   # compiler will re-write the asser to assert(not (5 == 6))
+##    assert(5 != 6)
+
+
+template `!=` (a,b: untyped): untyped =
+   not (a == b)
+
+assert(5 != 6)