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Diffstat (limited to 'doc')
| -rw-r--r-- | doc/README.md | 1 | ||||
| -rw-r--r-- | doc/primitive.md | 2 | ||||
| -rw-r--r-- | doc/swap.md | 30 |
3 files changed, 32 insertions, 1 deletions
diff --git a/doc/README.md b/doc/README.md index 4f8c49cf..0560a3a6 100644 --- a/doc/README.md +++ b/doc/README.md @@ -63,6 +63,7 @@ Primitives: - [Scan](scan.md) (`` ` ``) - [Search functions](search.md) (`⊐⊒∊`) - [Select](select.md) (`⊏`) +- [Self and Swap](swap.md) (`˜`) - [Self-search functions](selfcmp.md) (`⊐⊒∊⍷`) - [Shift functions](shift.md) (`»«`) - [Solo, Couple, and Merge](couple.md) (`≍>`) diff --git a/doc/primitive.md b/doc/primitive.md index 03634235..f2b47341 100644 --- a/doc/primitive.md +++ b/doc/primitive.md @@ -68,7 +68,7 @@ Functions that have significant differences from APL equivalents or don't appear Glyph | Name(s) | Definition | Description ------|--------------------|--------------------------------|--------------------------------------- `˙` | Constant | `{𝕩⋄𝕗}` | Return a function that returns the operand -`˜` | Self/Swap | `{𝕩𝔽𝕨⊣𝕩}` | Duplicate one argument or exchange two +`˜` | [Self/Swap](swap.md) | `{𝕩𝔽𝕨⊣𝕩}` | Duplicate one argument or exchange two `∘` | [Atop](compose.md) | `{𝔽𝕨𝔾𝕩}` | Apply `𝔾` to both arguments and `𝔽` to the result `○` | [Over](compose.md) | `{(𝔾𝕨)𝔽𝔾𝕩}` | Apply `𝔾` to each argument and `𝔽` to the results `⊸` | Before/Bind | `{(𝔽𝕨⊣𝕩)𝔾𝕩}` | `𝔾`'s left argument comes from `𝔽` diff --git a/doc/swap.md b/doc/swap.md new file mode 100644 index 00000000..0d9245e3 --- /dev/null +++ b/doc/swap.md @@ -0,0 +1,30 @@ +*View this file with results and syntax highlighting [here](https://mlochbaum.github.io/BQN/doc/swap.html).* + +# Self and Swap + +<!--GEN combinator.bqn +DrawComp ≍"˜" +--> + +Have the arguments to a function, but not in the right places? Self/Swap (`˜`) will fix it for you. There are only two APL-style 1-modifiers—that is, operands used as functions and applied to arguments—that make sense, and `˜` is both of them. It always calls its operand with two arguments: if there are two arguments to begin with, they are exchanged (Swap), and if there's only one, it's used for both arguments (Self). + +| Name | Call | Definition +|------|--------|:----------: +| Self | ` F˜𝕩` | `𝕩F𝕩` +| Swap | `𝕨F˜𝕩` | `𝕩F𝕨` + +Since `𝕩` is always the left argument, these two definitions can be unified as `{𝕩𝔽𝕨⊣𝕩}`, noting that [Left](identity.md) becomes a plain identity function when the left argument `𝕨` isn't given. + +Swap is arguably less transformative. Some common examples are `-˜` and `÷˜`, since these two functions run the [wrong way](../commentary/problems.md#subtraction-division-and-span-are-backwards) for BQN's evaluation order. This is very often useful in [tacit](tacit.md) programming, and less useful for explicit code. While it sometimes allows for shorter code by making a pair of parentheses unnecessary (say, `(a×b)-c` is `c-˜a×b`), I personally don't think this is always a good idea. My opinion is that it should be used when it makes the semantics a better fit for BQN, but putting the primary argument on the right and a secondary or control argument on the left. + + 'a' ⋈˜ 'b' + + " +" ⊏˜ 0‿1‿1‿0‿0≍1‿0‿1‿0‿1 + +Self re-uses one argument twice. In this way it's a little like [Over](compose.md), which re-uses one *function* twice. A common combination is with Table, `⌜˜`, so that the operand function is called on each combination of elements from the argument to form a square result. For example, `=⌜˜` applied to `↕n` gives the identity matrix of size `n`. + + ט 4 + + =⌜˜ ↕3 + +Note that Self isn't needed with Before (`⊸`) and After (`⟜`), which essentially have a copy built in: for example `F⊸G 𝕩` is the same as `F⊸G˜ 𝕩` by definition. |