Error Diagnostics

Every error in mino produces a structured diagnostic: a plain map with stable keys, a classified error code, source location, and a human-readable message. The same model serves the REPL, embedded hosts, and user code.

What You See

When an error occurs in the REPL or a script, mino renders a diagnostic with the error code, message, source file, line, column, and a snippet of the source with a caret pointer:

error[MTY001]: count: expected a collection, got int
  --> app.clj:18:3
   |
 18 |   (count 42)
    |   ^

Reader errors show the exact position:

error[MRE001]: unterminated string literal
  --> app.clj:7:8
   |
  7 | (def y "unterminated
   |        ^

Errors Are Data

In catch handlers, the exception value is always a map with these keys:

Key	Type	Meaning
`:mino/kind`	keyword	Category: `:reader`, `:syntax`, `:eval/type`, `:eval/arity`, `:eval/bounds`, `:user`, etc.
`:mino/code`	string	Stable error code, e.g. `"MTY001"`
`:mino/phase`	keyword	Processing phase: `:read`, `:eval`
`:mino/message`	string	Human-readable primary message
`:mino/data`	any	The original thrown value (for user exceptions)

Example: catching a type error and inspecting it.

(try
  (count 42)
  (catch e
    (println (:mino/kind e))    ;; :eval/type
    (println (:mino/code e))    ;; "MTY001"
    (println (:mino/message e)) ;; "count: expected a collection, got int"
    ))

throw and catch

(throw x) accepts any value. The catch handler always receives a diagnostic map, regardless of what was thrown. The original value is accessible via (ex-data e):

(try
  (throw "oops")
  (catch e
    (println (ex-data e))    ;; "oops"
    (println (ex-message e)) ;; "oops"
    (println (error? e))     ;; true
    ))

If you throw an ex-info map, ex-data and ex-message extract the payload transparently:

(try
  (throw (ex-info "not found" {:code 404}))
  (catch e
    (println (ex-data e))    ;; {:code 404}
    (println (ex-message e)) ;; "not found"
    ))

Helper Functions

Function	Description
`(error? x)`	True if `x` is a map with `:mino/kind`
`(ex-data e)`	Extract the data payload from a diagnostic map or ex-info
`(ex-message e)`	Extract the message from a diagnostic map or ex-info
`(last-error)`	Return the last diagnostic map, or nil
`(ex-info msg data)`	Create an exception map for throwing

Error Code Catalog

Every error has a stable code that can be matched programmatically. Codes are grouped by prefix:

Prefix	Category	Examples
`MRE`	Reader	Unterminated string, unexpected delimiter, unterminated collection
`MSY`	Syntax	Malformed `def`, `fn`, `let` special forms
`MNS`	Name resolution	Unresolved symbol, missing namespace alias
`MAR`	Arity	Wrong number of arguments
`MTY`	Type mismatch	Operation received unsupported value type
`MBD`	Bounds	Index out of range
`MCT`	Contract	Precondition or invariant violation
`MHO`	Host	Capability denied, host callback failure
`MIO`	I/O	`slurp`, `spit`, `sh`, file-system access failures
`MOM`	Out of memory	Allocator exhausted during a user-visible computation (e.g. bignum arithmetic)
`MOV`	Overflow	Integer overflow on `+` / `-` / `*` / `inc` / `dec` when the bignum-promoting variant (`+'` etc.) was not used
`MLM`	Limit	Step limit, heap limit, recursion depth exceeded
`MUS`	User	User-thrown exception
`MIN`	Internal	Runtime invariant violation or unhandled internal-state error

C API

Embedders have structured access to the last error:

/* Backward-compatible string access */
const char *mino_last_error(S);

/* Structured diagnostic (internal fields) */
const mino_diag *mino_last_diag(S);

/* Diagnostic as a mino map with :mino/* keys */
mino_val *mino_last_error_map(S);

/* Render to buffer in compact or pretty mode */
int mino_render_diag(S, diag, MINO_DIAG_RENDER_PRETTY, buf, sizeof(buf));

Existing code using mino_last_error() continues to work unchanged. The string is rendered from the structured diagnostic internally.