Expression¶

Status: Stable

documented, exercised by the test suite and/or worked examples, with no known limitations recorded.

Description¶

Expression

is a type specification in Read and ReadList that reads one complete Mathilda expression.

Examples (2)¶

Every input below was run against the current Mathilda build and its output recorded.

Applications (2)¶

Reads and evaluates one expression

In[1]:= s = OpenWrite["/tmp/mathilda_expr.txt"]; WriteString[s, "1 + 2*3"]; Close[s]; Read["/tmp/mathilda_expr.txt", Expression]
Out[1]= 7

One of the eight read types

In[2]:= MemberQ[{Byte, Character, Expression, Number, Real, Record, String, Word}, Expression]
Out[2]= True

Implementation notes¶

Definition. Expression is a type specification used by Read and ReadList. It has no builtin and no value — it is an inert token that tells the reader to parse one complete Mathilda expression from the stream. It is the default type when none is given (both read.c and readlist.c fall back to SYM_Expression).

Representation. A bare EXPR_SYMBOL (interned SYM_Expression). read.c's type_from_symbol maps it to the internal RT_EXPRESSION reader. The leaf is read unevaluated and the assembled result is then evaluated, so Read[s, Expression] evaluates the expression it read while Read[s, Hold[Expression]] keeps it in raw form.

Usage & limits. Meaningful only as a Read/ReadList type argument; evaluated on its own it returns itself. With no explicit type, ReadList["file"] reads every remaining expression. Past end of file the reader returns EndOfFile.

Attributes: none registered.

References¶

Source: src/io/read.c
Specification index: Mathilda_spec.md

Notes & additional examples¶

Notes¶

Expression is a type specification for Read and ReadList: it parses one complete Mathilda expression from the stream. It is the default read type — with no type given, ReadList["file"] reads every remaining expression.

The leaf is read unevaluated and the assembled result is then evaluated, so Read[s, Expression] evaluates what it read (the example above returns 7) while Read[s, Hold[Expression]] keeps the raw, unevaluated form. On its own Expression evaluates to itself; past end of file the reader returns EndOfFile.