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 BQN boasts of its functional capabilities, including first-class functions. What sort of functional support does it have, and how can a BQN programmer exercise these and out themself as a Schemer at heart?
 
-First, let's be clear about what the terms we're using mean. A language has *first-class functions* when functions (however they are defined) can be used in all the same ways as "ordinary" values like numbers and so on, such as being passed as an argument or placed in a list. Lisp and JavaScript have first-class functions, C has unsafe first-class functions via function pointers, and Java and APL don't have them as functions can't be placed in lists or used as arguments. This doesn't mean every operation is supported on functions: for instance, numbers can be added, compared, and sorted; while functions could perhaps be added to give a train, comparing or sorting them as functions (not representations) isn't computable, and BQN doesn't support any of the three operations when passing functions as arguments.
+First, let's be clear about what the terms we're using mean. A language has *first-class functions* when functions (however they are defined) can be used in all the same ways as "ordinary" values like numbers and so on, such as being passed as an argument or placed in a list. Lisp and JavaScript have first-class functions, C has unsafe first-class functions via function pointers, and Java 7 and APL don't have them as functions can't be placed in lists or used as arguments. This doesn't mean every operation is supported on functions: for instance, numbers can be added, compared, and sorted; while functions could perhaps be added to give a train, comparing or sorting them as functions (not representations) isn't computable, and BQN doesn't support any of the three operations when passing functions as arguments.
 
 Traditionally, APL has worked around its lack of first-class functions with operators, that is, second-order functions. Arrays in APL are first class while functions are second class and operators are third class, and each class can act on the ones before it. However, the three-tier system has some obvious limitations that we'll discuss, and BQN removes these by making every type first class.
 
@@ -80,13 +80,13 @@ To ← {
 ⟩
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-The term *functional programming* is more contentious, and has many meanings some of which can be vague. Here I use it for what might be called *first-class functional programming*, programming that makes significant use of first-class functions; in this usage, Scheme is probably the archetypal functional programming language. However, other definitions are also worth mentioning. APL is often called a functional programming language on the grounds that functions can be assigned and manipulated, and called recursively, all characteristics it shares with Lisp. I prefer the term *function-level programming* for this usage. A newer usage, which I call *pure functional programming*, restricts the term "function" to mathematical functions, which have no side effects, so that functional programming is programming with no side effects, often using monads to accumulate effects as part of arguments and results instead. Finally, *typed functional programming* is closely associated with pure functional programming and refers to statically-typed functional languages such as Haskell, F#, and Idris (the last of which even supports *dependently-typed functional programming*, but I already said "finally" so we'll stop there). Of these, BQN supports first-class functional and function-level programming, allows but doesn't encourage pure functional programming, and does not support typed functional programming, as it is dynamically and not statically typed.
+The term *functional programming* is more contentious, and has many meanings some of which can be vague. Here I use it for what might be called *first-class functional programming*, programming that makes significant use of first-class functions; in this usage, Scheme is probably the archetypal functional programming language. However, other definitions are also worth mentioning. APL is often called a functional programming language on the grounds that functions can be assigned and manipulated, and called recursively, all characteristics it shares with Lisp. I prefer the term *function-level programming* for this usage. A newer usage, which I call *pure functional programming*, restricts the term "function" to mathematical functions, which have no side effects, so that functional programming is programming with no side effects, often using monads to accumulate effects as part of arguments and results instead. Finally, *typed functional programming* is closely associated with pure functional programming and refers to languages influenced by type theory such as Haskell, F#, and Idris (the last of which even supports *dependently-typed functional programming*, but I already said "finally" so we'll stop there). Of these, BQN supports first-class functional and function-level programming, allows but doesn't encourage pure functional programming, and does not support typed functional programming, as it's dynamically and not statically typed.
 
 Another topic we are interested in is *lexical scoping* and *closures*. Lexical scoping means that the realm in which a variable exists is determined by its containing context (in BQN, the surrounding set of curly braces `{}`, if any) within the source code. A closure is really an implementation mechanism, but it's often used to refer to a property of lexical scoping that appears when functions defined in a particular block can be accessed after the block finishes execution. For example, they might be returned from a function or assigned to a variable outside of that function's scope. In this case the functions can still access variables in the original scope. I consider this property to be a requirement for a correct lexical scoping implementation, but it's traditionally not a part of APL: implementation might not have lexical scoping (for example, J and I believe A+ use static scoping where functions can't access variables in containing scopes) or might cut off the scope once execution ends, leading to value errors that one wouldn't predict from the rules of lexical scoping.
 
 ## Functions in APL
 
-This seems like a good place for a brief and entirely optional discussion of how APL handles functions and why it does it this way. As mentioned above, APL's functions are second class rather than first class. However, it's worth noting that the barriers to making functions first-class objects have been entirely syntactic and conceptual, not technical. In fact, the J language has for a long time had [a bug](http://www.jsoftware.com/pipermail/programming/2013-January/031260.html) that allows an array containing a function to be created: by selecting from the array, the function itself can even be passed through tacit functions as an argument!
+This seems like a good place for a brief and entirely optional discussion of how APL handles functions and why it does it this way. As mentioned above, APL's functions are second class rather than first class. But the barriers to making functions first-class objects have been entirely syntactic and conceptual, not technical. In fact, the J language has for a long time had [a bug](http://www.jsoftware.com/pipermail/programming/2013-January/031260.html) that allows an array containing a function to be created: by selecting from the array, the function itself can even be passed through tacit functions as an argument!
 
 The primary reason why APL doesn't allow functions to be passed as arguments is probably syntax: in particular, there's no way to say that a function should be used as the left argument to another function, as an expression like `F G x` with functions `F` and `G` and an array `x` will simply be evaluated as two monadic function applications. However, there's no syntactic rule that prevents a function from returning a function, and Dyalog APL for example allows this (so `⍎'+'` returns the function `+`). Dyalog's `⎕OR` is another interesting phenomenon in this context: it creates an array from a function or operator, which can then be used as an element or argument like any array. The mechanism is essentially the same as BQN's first class functions, and in fact `⎕OR`s even share a form of BQN's [syntactic type erasure](../commentary/problems.md#syntactic-type-erasure), as a `⎕OR` of a function passed as an operand magically becomes a function again. But outside of this property, it's cumbersome and slow to convert functions to and from `⎕OR`s, so they don't work very well as a first-class function mechanism.
 
@@ -107,9 +107,9 @@ First, let's look at the basics: a small program that has functions as its argum
       v0 + ((𝕏 1) - v0) × ⊢
     }
 
-We can pass it the exponential function as an argument by giving it the name `Exp` and then referring to it in lowercase (that is, in a subject role). The result is a train that adds 1 to *e*-1 times the argument.
+We can pass it the [exponential](arithmetic.md#basic-arithmetic) function as an argument by giving it the name `Exp` and then referring to it in lowercase (that is, in a subject role). The result is a [train](train.md) that adds 1 to *e*-1 times the argument.
 
-        Lin ← { v0←𝕏0 ⋄ v0+((𝕏1)-v0)×⊢ }
+        Lin ← { v0←𝕏0 ⋄ v0+((𝕏1)-v0)×⊢ }  # (copy of above)
         Exp ← ⋆
         Lin exp
 
@@ -135,12 +135,12 @@ Note also in this case that we could have used a modifier with a very similar de
 
 Its call syntax is simpler as well. In other cases, however, the function version might be preferable, for example when dealing with arrays of functions or many arguments including a function.
 
-        _lin ↩ { v0←𝔽0 ⋄ v0+((𝔽1)-v0)×⊢ }
+        _lin ↩ { v0←𝔽0 ⋄ v0+((𝔽1)-v0)×⊢ }  # (copy again)
         Exp _lin 5
 
 ### Arrays of functions
 
-It's very convenient to put a function in an array, which is fortunate because this is one of the most important uses of functions as subjects. Here's an example of an array of functions with a reduction applied to it, composing them together.
+It's very convenient to put a function in an array, which is fortunate because this is one of the most important uses of functions as subjects. Here's an example of an array of functions with a [fold](fold.md) applied to it, composing them together.
 
         {𝕎∘𝕏}´ ⋆‿-‿(ט)