Change spec so function and left arg are still evaluated if right arg is nothing

1 files changed, 2 insertions, 2 deletions

diff --git a/spec/evaluate.md b/spec/evaluate.md
index 575558ff..3d1ea1c5 100644
--- a/spec/evaluate.md
+++ b/spec/evaluate.md
@@ -10,7 +10,7 @@ Here we assume that the referent of each identifier, or equivalently the connect
 
 The result of parsing a valid BQN program is a `PROGRAM`, and the program is run by evaluating this term.
 
-A `PROGRAM` or `BODY` is a list of `STMT`s (for `BODY`, the last must be an `EXPR`, a particular kind of `STMT`), which are evaluated in program order. The statement `nothing` does nothing when evaluated, while `EXPR` evaluates some APL code and possibly assigns the results, as described below.
+A `PROGRAM` or `BODY` is a list of `STMT`s (for `BODY`, the last must be an `EXPR`, a particular kind of `STMT`), which are evaluated in program order. The statement `EXPR` evaluates some APL code and possibly assigns the results, while `nothing` evaluates any `subject` or `Derv` terms it contains but discards the results.
 
 A block consists of several `BODY` terms, some of which may have an accompanying header describing accepted inputs and how they are processed. An immediate block `brImm` can only have one `BODY`, and is evaluated by evaluating the code in it. Other types of blocks do not evaluate any `BODY` immediately, but instead return a function or modifier that obtains its result by evaluating a particular `BODY`. The `BODY` is identified and evaluated once the block has received enough inputs (operands or arguments), which for modifiers can take one or two calls: if two calls are required, then on the first call the operands are simply stored and no code is evaluated yet. Two calls are required if there is more than one `BODY` term, if the `BODY` contains the special names `𝕨𝕩𝕤𝕎𝕏𝕊`, or if its header specifies arguments (the header-body combination is a `_mCase` or `_cCase_`). Otherwise only one is required.
 
@@ -18,7 +18,7 @@ To evaluate a block when enough inputs have been received, first the correct cas
 
 The only remaining step before evaluating the `BODY` is to bind the inputs and other names. Special names are always bound when applicable: `𝕨𝕩𝕤` if arguments are used, `𝕨` if there is a left argument, `𝕗𝕘` if operands are used, and `_𝕣` and `_𝕣_` for modifiers and combinators, respectively. Any names in the header are also bound, allowing multiple assignment for arguments.
 
-If there is no left argument, but the `BODY` contains `𝕨` at the top level, then it is conceptually re-parsed with `𝕨` replaced by `·` to give a monadic version before application. As the only effect when this re-parsed form is valid is to change some instances of `arg` to `nothing`, this can be achieved efficiently by annotating parts of the AST that depend on `𝕨` as conditionally-nothing. However, it also causes an error if `𝕨` is used as an operand or list element, where `nothing` is not allowed by the grammar.
+If there is no left argument, but the `BODY` contains `𝕨` at the top level, then it is conceptually re-parsed with `𝕨` replaced by `·` to give a monadic version before application; this modifies the syntax tree by replacing some instances of `arg` with `nothing`. However, it also causes an error if, in a function that is called with no left argument, `𝕨` is used as an operand or list element, where `nothing` is not allowed by the grammar. The same effect can also be achieved dynamically by treating `·` as a value and checking for it during execution. If it is used as a left argument, then the function should instead be called with no left argument (and similarly in trains); it it is used as a right argument, then the function and its left argument are evaluated but rather than calling the function `·` is "returned" immediately; and if it is used in another context then it causes an error.
 
 ### Assignment