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1:- :~ navhome/'/docs/'
2 sort/'6'
3 title/'Hoon Errors'
4 ==
5;>
7# Hoon Errors
9Ideally your Hoon works perfectly the first time. But...
11## Syntax errors
13When you get a syntax error, you'll see a message like
15```
16syntax error: 10 12
17```
19This is a line and column number; more exactly, the line and
20column of the first byte the parser couldn't interpret as part of
21a correct Hoon file. These values are always correct.
23Usually, the line and column tell you everything you need to
24know. But the worst-case scenario for a syntax error is that,
25somewhere above, you've confused Hoon's tall form by using the
26wrong fanout for a rune. For example, `%+` ([*cenlus*](../rune/cen/lus),
27a function call whose sample is a cell) has three subhoons:
29```
30%+ foo
31 bar
32baz
33```
35But if you make a mistake and write
37```
38%+ foo
39bar
40```
42the parser will eat the next hoon below and try to treat it as a
43part of the `%+`. This can cause a cascading error somewhere
44below, usually stopped by a `==` or `--`.
46When this happens, don't panic! Binary search actually works
47quite well. Any hoon can be stubbed out as `!!`. Find the
48prefix of your file that compiles, then work forward until
49the actual error appears.
51## Semantic errors
53Now your code parses but doesn't compile.
55### Turn on debugging
57Your first step should be to put a `!:` ("zapcol") rune at the
58top of the file. This is like calling the C compiler with `-g`;
59it tells the Hoon compiler to generate tracing hoons.
61Bear in mind that `!:` breaks tail-call optimization. This is a
62bug, but a relatively low-priority bug. `!.` turns off `!:`.
63Note that `!:` and `!.` are hoon-level, not file-level; you can
64wrap any hoon in either.
66### Error trace
68If you have `!:` on, you'll see an error trace, like
70```
71/~zod/home/0/gen/hello:<[7 3].[11 21]>
72/~zod/home/0/gen/hello:<[8 1].[11 21]>
73/~zod/home/0/gen/hello:<[9 1].[11 21]>
74/~zod/home/0/gen/hello:<[10 1].[11 21]>
75/~zod/home/0/gen/hello:<[11 1].[11 21]>
76/~zod/home/0/gen/hello:<[11 7].[11 21]>
77nest-fail
78```
80The bottom of this trace is the line and column of the hoon which
81failed to compile, then the cause of the error (`nest-fail`).
83Hoon does not believe in inundating you with possibly irrelevant
84debugging information. Your first resort is always to just look
85at the code and try to figure out what's wrong. This practice
86strengthens your Hoon muscles.
88(Consider the opposite extreme; imagine if you had a magic bot
89that always fixed your compiler errors for you. Pro: no time
90wasted on compiler errors. Con: you never learn Hoon.)
92## Common errors
94Moral fiber is all very well and good, but sometimes you're
95stumped. Couldn't the compiler help a little? These messages do
96mean something. Here are the three most common:
98### `nest-fail`
100This is a type mismatch (`nest` is the Hoon typechecker). It
101means you tried to pound a square peg into a round hole.
103What was the peg and what was the hole? Hoon doesn't tell you by
104default, because moral fiber, and also because in too many cases
105trivial errors lead to large intimidating dumps. However, you
106can use the `~!` rune ([*sigzap*](../rune/sig/zap)) to print the type of
107any hoon in your stack trace.
109For instance, you wrote `(foo bar)` and got a `nest-fail`. Change
110your code to be:
112```
113~! bar
114~! +6.foo
115(foo bar)
116```
118You'll get the same `nest-fail`, but this will show the type of
119`bar`, then the type of the sample of the `foo` gate.
121### `find.foo`
123A `find.foo` error means limb `foo` wasn't found in the subject.
124In other words, "undeclared variable."
126The most common subspecies of `find` error is `find.$`, meaning
127the empty name `$` was not found. This often happens when you
128use a hoon that does not produce a gate/mold, as a gate/mold. For
129instance, `(foo bar)` will give `find.$` if `foo` is not actually a
130function.
132### `mint-vain` and `mint-lost`
134These are errors caused by type inference in pattern matching.
135`mint-vain` means this hoon is never executed. `mint-lost` means
136there's a case in a `?-` ([*wuthep*](../rune/wut/hep)) that isn't handled.
138## Runtime crashes
140If your code crashes at runtime or overflows the stack, you'll
141see a stack trace that looks just like the trace above. Don't
142confuse runtime crashes with compilation errors, though.
144If your code goes into an infinite loop, kill it with `ctrl-c` (you'll
145need to be developing on the local console; otherwise, the
146infinite loop will time out either too slowly or too fast). The
147stack trace will show what your code was doing when interrupted.
149The counterpart of `~!` for runtime crashes is `~|`
150([*sigbar*](../rune/sig/bar)):
152```
153~| foo
154(foo bar)
155```
157If `(foo bar)` crashes, the value of `foo` is printed in the
158stack trace. Otherwise, the `~|` has no effect.
160## Debugging printfs
162The worst possibility, of course, is that your code runs but does
163the wrong thing. This is relatively unusual in a typed
164functional language, but it still happens.
166`~&` ([*sigpad*](../rune/sig/pad)) is Hoon's debugging printf.
167This pretty-prints its argument:
169```
170~& foo
171(foo bar)
172```
174will always print `foo` every time it executes. A variant is
175`~?` ([*sigwut*](../rune/sig/wut)), which prints only if a condition is
176true:
178```
179~? =(37 (lent foo)) foo
180(foo bar)
181```
183For now, you need to be on the local console to see these debug
184printfs (which are implemented by interpreter hints). This is a
185bug and, like all bugs, will be fixed at some point.