diff options
Diffstat (limited to 'artima/scheme/scheme11.ss')
| -rw-r--r-- | artima/scheme/scheme11.ss | 10 |
1 files changed, 5 insertions, 5 deletions
diff --git a/artima/scheme/scheme11.ss b/artima/scheme/scheme11.ss index 4063ed7..96214eb 100644 --- a/artima/scheme/scheme11.ss +++ b/artima/scheme/scheme11.ss @@ -15,8 +15,8 @@ a C-like ``for`` loop and I said that it was suffering from the problem of multiple evaluation. Here I explain what the problem is and how to cure it. In order to understand the issue, you must always remember that macros *expand* code at compile time, -but they not *evaluate* it: that means that pattern variables do *not* -correspond to evalued expression, as ordinary variables, but they +but they not *evaluate* it: this means that pattern variables do *not* +correspond to evaluated expression, as ordinary variables, but they correspond to expressions to be evaluated later, at runtime. As a consequence, it is easy to write macros @@ -37,7 +37,7 @@ with a computation:: (printf "computing the value of end\n") 3) -Then our naive macro suffers for the multiple evaluation problem:: +Then our naive macro suffers from the multiple evaluation problem:: > (for i 0 (get-end) 'do-nothing) computing the value of end @@ -103,7 +103,7 @@ $$repeat-benchmark: I took the number ``n`` from the command line arguments in order to fool the compiler: if I hard coded ``(+ 1 1)``, the compiler would replace it with 2 at compilation time, therefore not performing -the computation! (in the original version of this episode I made that +the computation! (In the original version of this episode I made that mistake, thanks to Aziz Ghuloum for pointing it out). The output of the script is the following:: @@ -152,7 +152,7 @@ from the macros we have defined until now. The reason why the ``test`` macro is different is that it expands into a ``lambda``-expression and therefore the arguments of the macro are evaluated only when the ``lambda`` function is called -and not a definition time. In other words, we are using a pattern of +and not at definition time. In other words, we are using a pattern of *delayed evaluation* here. This is important, since we want to distinguish the definition of a test from its execution. For instance, let me define a trivial test:: |